Having completed 5 of 18 trophies (reaching tier 1 of the Spring Cleaning badge) granted the Steam user a Spring Cleaning background, 3 emotes, and a mystery box. Description of the box states:

This item might be useful in a future sale.
Earned for participating in the Spring Cleaning Event in 2018.

Any guesses? Personally, I predict the mystery box will relate to Valve's new card game, Artifact, and perhaps Steam users who have the box may obtain an Artifact card or a card pack. Why cards? Well, Valve has released items with price tags for other games the company has developed. CSGO has skins, Team Fortress 2 has the hats, Dota 2 has market items on the community market (I am vague on what Dota 2 has). I cannot deny Artifact might have marketable cards in the future. Even though the description mentions the box might be useful in a future sale does not indicate the item must relate to a sale; just that an event coinciding with a Steam sale is possible such as if Valve released extra information about Artifact during the summer sale for example.

Edit: To add to my prediction, Artifact on the Steam store has a 2018 release date.
2018 Spring Cleaning event page
Spring Cleaning market items
Mystery box listing

Edit: The Mystery Box contained cosmetics for your alien in the Summer Sale 2018 minigame called Saliens.
https://steamcommunity.com/saliengame/play/

6 years ago*

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Maybe additional xx% off for any purchase?

Maybe its like a lootbox that will give you any steam game. (doubt that, but it would be cool)

6 years ago
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Maybe additional xx% off for any purchase?

This was my thought too. A "special" coupon that adds +some % on any existing sale.

6 years ago
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This also crossed my mind, with an immediate follow-up along the lines of "...which might mean we'll get shittier discounts to compensate for the coupon"

6 years ago
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A lootbox? Sure, but you gotta buy a key first to open it - introduced at the next big sale :P

6 years ago
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It's a coupon.

6 years ago
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yeah, nothing exciting or new

6 years ago
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I'm gonna take a wild guess that it's a universal coupon that can be used only once or a limited amount of times. Like -15% on top of whatever the sale price already is or something like that. But honestly, it's probably related to another badge or (boring) stuff like that.

6 years ago
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I would love an universal coupon. Especially if I can get a good price on something like Ni no Kuni II.

6 years ago
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+1 to this! I am waiting in anticipation for a sale on that one xD

6 years ago
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A pack of cards for Steam Summer Sale 2018

6 years ago
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I don't think it will be a pack but definitely 1 card

6 years ago
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Probably another box inside :3

6 years ago
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6 years ago
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It's a companion cube

6 years ago
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or a cosmic cube

6 years ago
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i'd rather prefer the Eye of Agamotto

6 years ago
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It'll give a collectible figure, when you collect certain amount of figures you can trade them for discount coupons and even games.

6 years ago
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What's in the box?:)

A pair of new Socks. :)

6 years ago
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People are listing it, I can't list it, sad on me.

For me, some old game or 3 cards for the next summer event.

6 years ago
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The mystery box was marketable, but Valve disabled it. The feature was not intended.

Here is a response from a Valve employee: https://www.reddit.com/r/Steam/comments/8lue1l/got_this_after_completing_all_the_projects_wonder/dzii3vl/

6 years ago*
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Soon as I receive the box I check if it can be solded on market but no, now I saw this and I was like; wtf - but good to know.

Edit: Oh thanks for the Reddit

6 years ago
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6 years ago
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a cat

6 years ago
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Maybe a Gordon Freeman lookalike cat.

6 years ago
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It's probably chocolate!

I'd be amused if Steam would determine the content of the box by how serious the account owner was involved in their spring cleaning.
Example:
If you just started and closed games, you'd get some rather useless coupon.
If you actually invested time and got plenty of achievements, you'd get a 10€ voucher, only usable during the summer sale.

They'd avoid that people cheat (knowingly), as all qualifying events would be a part of the past, once when they share the rewards.

6 years ago
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That would be a pretty neat idea, and a kick in the face for a handful of us (including me).

6 years ago
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It would include me too.
I imagine though that they wouldn't want to disappoint us too much, in such a scenario. ;)

6 years ago
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Then they would be pretty crazy, expecting people to play like 16 full games in 4 days.
Even if people did like me and I just picked the really short games to try, complete, then idle for cards. 0_o

6 years ago
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I didn't suggest that people would have had to fully play several games.
They could either use fixed requirements as x hours played in total etc. or they could reward the top 10% of the most active players with the best prize category, the next y% with the second best rewards category and so on. Nothing crazy about it, just something new.

6 years ago
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its mulit coupon not 1 >.> its be 90% 50% 33% 10%

6 years ago
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Half Life 3.

6 years ago
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a cake... who's also a lie

Im with the coupon option too :S

6 years ago
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living cat

6 years ago
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Something disappointing.

6 years ago
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6 years ago
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+1

6 years ago
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The head of Tracy Mills!! (from the movie Seven) O.O

In all seriousness, probably a coupon. Knowing Valve, it will probably be for Artifact.;)

6 years ago
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A teaser trailer for HL3, coming summer 2028 (no relation with the Half-Life series, the letters stand for something else).

6 years ago
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a 99% off coupon for a $80 special edition game, so people can come to the forums and complain how they have no money to buy it. T_T

6 years ago
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6 years ago*
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I beat you by an hour.;)

Although I am surprised that no mentioned that scene right away! LOL!

6 years ago
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6 years ago
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That could very well be! I am turning 45 on June 22nd, after all!;)

6 years ago
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6 years ago
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But wouldn't that have made you 28?;) ;)

6 years ago
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Dissapointment :3

6 years ago
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An item to be used in their pixel point and click.

6 years ago
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A hat obviously

6 years ago
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Pain.

6 years ago
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