From the above, I'd definitely recommend:
Swarm is decent. I think it has some problems--levels too long, overall game too short, not well explained, scoring system doesn't really make much sense, but it's sort of neat to speed-run and it never gets old watching your little guys get sliced, diced, skewered, or zapped. It's definitely worth $2.50. 9/12 achievements just require finishing game, one requires some post-game play, and two require particular score attack stuff.
Quarrel is basically like Risk only instead of rolling dice you play Scrabble against your opponent. It works really well, but you might find it quite hard if you don't have a strong vocabulary. Playing even the easier AIs requires a pretty good sense for anagram games. Achievements will essentially require cheating or perfect, super-human play; I wrote an AI to play for me once I was finished playing the game as far as I could.
Mercury HG is vastly superior to the PSP and Wii versions, and works really well with custom soundtracks; the levels pulse and vibrate with the music playing. Both DLC packs are dirt cheap too. One of the best frustrating challenge games on XBLA. Achievements simply require completion of game, but very difficult.
World of Keflings is a worthy sequel to Kingdom of Keflings. Both games are basically non-violent RTSes. Imagine an RTS where you only had to deal with resource management and refining and building. Very relaxed, happy, upbeat. New DLC is coming out soon too. Easy achievements.
Costume Quest is short and sweet and relatively shallow, but it's got excellent writing, a great visual style, and reminds me of a children's version of Mario RPG's battle system and Earthbound's tone and sense of exploration. Both the game and the DLC are worth it. Easy achievements.
Stacking is an excellent adventure game. Again very funny writing, but the best part about it is that every puzzle in the game has multiple solutions (2-6 solutions). You only need to pick one solution, sure, but the real fun is in discovering them all. They're each individually simple--the game has no permanent inventory, so the different character abilities focus as a contextually valid set of options for solving the problem--but in aggregate it ends up being pretty substantial. Warning, the framerate and motion blur can get a bit disorienting if you're sick. The DLC is about one-third the length of the game if you finish the game and are wondering about the DLC. Easy achievements.
