The Bard's Tale is pretty fun, especially as you're playing on a Tablet. Piggin' awkward on a small phone screen, though!
Mass Effect Infiltrator was wicked fun!!! A friend used to come round to play Mass Effect on my PC, so while he did that I'd kick back on the sofa and play this on my phone. Tablet should be brilliant!!
Spy Mouse kept me entertained for weeks on a casual, 10-15 free minutes here and there basis.
Judge Dredd is simple but pretty good.
StarPagga is basic, but solid fun with reasonable replay value.
Predators is just addictive!
Pool Break Pro - Pass a good half-hour. More if you're into your snooker/pool-type games.
Backstab HD - Good if you like Assassins Creed and are good with a touchscreen.
If you can get WiFi:
Blood And Glory - Roman Gladiators!!!
Dark Legends - Silly 'Vampire The Masquerade' type MORPG thing, yet strangely addictive.