This one is a bit late, so if you’re reading this, immediatly
Frozen Synapse: awesome little tactical game with a surprisingly solid single player. Comes with a copy to gift to a friend!
Trine: cute puzzler for $2, worth it.
Mount and Blade: only buy Warband! It’s the most up-to-date one and on sale individually for cheaper today. This game is weird and entertaining as hell. A sort of mishmash of RPG, strategy, and nation-building. Includes the most accurate jousting this side of no other game involves jousting so you know.
Fallout: New Vegas. By all accounts, $15 will get you dozens of hours of very well regarded wasteland wandering adventure.
Bionic Commando Rearmed
DROP IT LIKE IT’S A SHITTY GAME