Can't beat GalCivII on a laptop!
short chunks - yes
no cd - yes
strategy - yes


... and presumably...
no extra investment - yes!

Ever try Warcraft? Google it and you'll find a host of suggestions.

Solitaire, Hearts, Minesweeper...

Ever try Age of Wonders II: Shadow Magic? Great Fantasy Strategy game with a little RPG to it, you'd have to get a no CD crack though to bypass having the CD in the drive...

There are a couple of good games for download like Dominions III and some others, I am not that familiar with them so not sure what to reccommend.

You can download Steel Panther's World at War and/or Steel Panther's: Main Battle Tank (or even Steel Panther's WW2) all tactical war games based on the original Steel Panthers game from the nineties, but updated with individual twists, and most importantly free to download. Check out Shrapnel games or Matrix games for these or other possibilities.

And as Newbyl said there is a decent little game called Galactic Civilizations II that is supposed to be fun to play.

I am addicted to PuzzleQuest, although I haven't gotten it for the PC yet, mostly because my addiction is bad enough playing it on the DS.

Heh. Got Galciv (1 and 2 and all expansions), of course. Have Puzzle Quest for 360.
@Firebender I guess the original Warcraft would work... But 3 certainly needs the CD.

I would suggest Darwinia or DEFCON, both by Introversion Software.

Neither requires the CD, both are RTS games (both have won numerous awards, Darwinia winning the 2006 grand prize at the Independent Game Awards). Both can actually run off of a USB drive if you really want to (I keep a copy of both on a thumb drive in case I'm ever stuck between classes with nothing to do).

More good suggestions, Feud. Fortunately, I own both.

You could also get an old copy of Master of Orion 2. They can be pricey these days, but still fun!

Heh. Have MOO2 as well. Love that game.

How about one of the Worms titles?