Plants Vs. Zombies works pretty well for that sort of situation, and is a lot of fun. And as JohnnyDelRay said, FPS games have a long enough history that you should be able to find one you can run. I pretty frequently play FPS's on break in my night classes, even hopping online for a round or two if the break is long enough.
Emulators would be a safe bet, if it weren't for the fact that you're almost required to pirate the games to have anything to run on them -- I guess if you legally have some DOS games, Dosbox counts as an emulator. The latest version has been reworked to actually play nice with the Windows 7 version of Aero, so there's that.
Fallout 1+2 will run on pretty much any computer made in the last 15 years, and KoTOR is old enough that most laptops can run it. Mine maxes every setting but the anti-aliasing out quite nicely, and it's hardly a gaming laptop. The Civilization series is also a pretty safe bet -- and it's been running for so long that, regardless of your computer's capabilities, there should be a Civ game for you. You could also try an Elder Scrolls game, if you want. If you can't run Oblivion, Morrowind should fit the bill, and Daggerfall is legally free now, not to mention one of the best games of all time. Betrayal at Krondor is another great DOS era RPG that was released as legal freeware.
Finally, if you're into 4x games, Anacreon [http://www.neurohack.com/anacreon/] is one of the oldest, and one of the best. The source of the DOS version is still being maintained, and I'm pretty sure the current free version is actually a 32 bit compile that works just fine on modern versions of Windows -- it certainly worked in XP, with no need for Dosbox or VDM sound. There's also a remake available on the same site, with better graphics but less detailed gameplay.