My current commuter rotation has been Killzone Mercenary, Hot Shots Tennis, Luftrausers and Child of Light. I got the first two on a PSN sale for cheap; can't believe how amazing Killzone looks and plays on the Vita, very close to the PS3 games. Also Child of Light really grew on me. At first it's awesome then I started to doubt if the game mechanics were just fluff but the difficulty came back around and I think it's a really awesome JRPG system. I like using the touch screen for the firefly guy and the artwork is just stunning on the Vita OLED screen.
The Dota 2 International 4 starts this weekend so I'll probably be on Steam watching that and hopping into games.