Having a (late) look through your library, I can recommend either Superbrothers: Sword & Sworcery or Scribblenauts Unlimited as both memorable and short/casual games.

If you're in a less "serious" mindset (seeing as you're doing SR4 at the moment), then you might want to start with Scribblenauts - trying off-kilter solutions to straight-faced puzzles/problems is an easy recipe to create memorable moments (my personal favourite was when I thought of writing up a robot knight, then have him riding a jet-propelled dinosaur while wielding a magical rocket launcher - in order to defend a fire station from a zombie outbreak).

Achievement-wise, it took me about 13~ hours to 100% it, although I bit the bullet by the end and checked some of the more difficult star shard solutions online; depending on how fluent you are in English it might take anywhere from 9 to 15 hours to fully complete.

If you want a change of pace though, you should give Superbrothers: Sword & Sworcery a go - it's infinitely more minimalistic and slow-paced, much more focused on exploration rather than action and tries some interesting things with narration and exposition; very memorable if you can get into its' "groove".

However (and this is a big however), if you don't feel invested in the game after the first 20-30 minutes, you're better off dropping it, as it is both very short and focused on this particular aesthetic.

You can probably 100% this game in as little as 4 hours (I think I have an overall total of 4,5 hours), although there is one achievement that requires the player to complete a chapter on a specific real-world date ("#honest", as the game follows real-world lunar movements and requires a specific phase of the moon to complete said chapter), so you might want to check around about that particular achievement beforehand.

I usually display those achievements I've had most fun completing (like "Foxiest of the Hounds" for Deus Ex: HR, or "Dungeon Runner" for Legend of Grimrock) - since I just *love* collecting stuff (probably owning to some minor form of OCD, dunno), these are usually 100% achievements.

I kinda liked @Second's idea though, although I'll be damned if I have any clue as to what the message should say :P.

Taking a look through your library, I can see a couple that are "easy" to 100%, mainly that they do not require any serious sidetracking or grinding:

*Thomas Was Alone
Took me about 3 or so hours to complete 100%, in a single run - no real optionals except maybe "Part of the Problem", which you'll probably get by accident anyway. The "X" amount of Jumps achievements will probably unlock while you attempt to solve the game's puzzles, as will the 100 Deaths one.

*Superbrothers: Sword & Sworcery
Again, only about 4 hours to 100% completion but the problem here is that for two of the achievements you'll need to replay the final chapter - the first time by waiting a set amount of real-world days for the Moon to hit specific phases (you just need to start up the game on the specific date) for the "#honest" achievement and the second time to change your PC's date manually to reach the same Moon phase for the "Cheating Cheater" achievement. Check this forum post for more details.

*Deus Ex: Human Revolution - The Missing Link
Another relatively "easy" one, depending on how good you are with stealth. You'll probably need about 2 playthroughs total - one for "Factory Zero" (finish the game with no upgrades) and one for all the rest. "Out of the Frying Pan" and "Never Stop Looking" are story-based and thus, unmissable, "That Old Adage" (needs CASIE upgrade), "Apex Predator" (needs good stealth skills), "All of the Above" (needs Move Heavy Objects and Jump Enhancement upgrades) and "Good Samaritan" are either optional main mission options or side-quests, "The Learn'd Scholar", "Back Stage Pass" and "Never Forget" are exploration-based achievements.

*Costume Quest
For the most part, requires light exploration to find all the secrets (but allows backtracking, in case you miss anything). The only "problematic" achievement is "Tisn't The Season", for which you'll have to fire up the game during the 25th of December (Christmas Day).

Note that the final five achievements are part of the "Grubbins on Ice" DLC, included in the game but played seperately from the main campaign.

*Portal 2
You might want to ask around for a co-op partner for the ones you're missing, with the exception of "Still Alive" and "Talent Show" which are a bit tricky unless you've gotten used to the co-op campaign level layouts and solutions, the rest are easy enough to do in a single playthrough (about 1-4 hours, depends on whether either of the players has done the co-op campaign before).

*Deus Ex: Human Revolution
If you can stomach running the game again for a no-kill, no-detect run on Give me Deus Ex difficulty (easier than it sounds, you are allowed quicksaves), you can also do "Cloak & Daggers" (check a walkthrough for specifics), "Doctorate" (this list here[deusex.wikia.com] is very useful) and "Gun Nut" (fully upgrade the tranquilizer rifle, which you'll need for a no-kill run) on the way.

-- If you're feeling lucky, give Poker Night At The Inventory a try - it's very much luck-based so I can't really give an estimate on time, I've done about 75% of the achievements in 3-ish hours though.
-- Alternatively, try Shank if you prefer platform games, you can get the entire achievement list except "Backstory" (needs co-op) if you do a playthrough on Hard difficulty.
-- You're also missing the "Speed Run" achievement in Braid, you can either replay the game or check around on YouTube for a few speedruns to get the feel of the levels before trying it (if memory serves, you must beat the game in under 45 minutes).
-- Finally, if you are good at puzzle games, Splice might be a good option, as half the achievements are for completing puzzles and the other half for completing the Epilogue puzzles (unlocked when doing main puzzle strands with "Angelic" solutions (i.e. in less than the maximum moves allowed)).

Regards,
GreyGore