How about Civilization 3? Easy to run even for older laptops, it's pretty tolerant of alt-tabbing and offers nearly endless amount of entertainment (assuming you like it obviously

). Personally I play my Civ games by playing past the 2050 AD and with all victory modes off - ie. going for total world domination. Will provide ya with ridiculous amounts of play time, single game could keep you going through a whole semester depending on how you play
Also, yeah, if you have someone to play with (or even on your own, but it's much, MUCH more fun with a real person rather than the CPU), go for Worms: Armageddon... or Worms Reloaded I suppose, I just I prefer Armageddon myself (can change more options, don't have the weapon box covering your whole damn screen etc.). Don't need internet for it, can play Hotseat or in LAN (just bring an Ethernet cable, they're quite cheap)