Warcraft 3 Frozen Throne, and get the custom campaign called The Chosen Ones. Work of utter beastliness, unbelievably addictive too!
As others have mentioned Civ 4 (or 5 if you can get the steam offline mode to work reliably for you) is always a good way to pass hours offline. Last time I was traveling I worked my way through Borderlands and replayed early "Final Fantasy"s.
+1 to the Torchlight suggestions. Depends what you're into though, could range from Skyrim to Minecraft to Civilization.
Kerbal Space Program, spend hours blowing up rockets, doing orbital manoeuvres, trying to land on the Mun and maybe even fly a space plane or two. Get the demo from here though I'm not sure how much it contains having bought it ages ago.
If you're still looking for suggestions, Sword of the Stars 1, that could eat up some time. Baldurs Gate 2 and most other RPGs, Anno 2070, SimCity 4 - not Societies, god stay away from Societies +1 for Civ games, I prefer Civ 4 myself. +1 for Torchlight. Build a city in Minecraft!
Another +1, Civ 4 was just an all round more pleasant experience (plus Leonard Nimoy does the voice overs and the little videos of the wonders being built were brilliant, why weren't they kept ffs )
I am highly disappointed noone has reccomend the original KOTOR. I've lost days to that game, all offline, completely without activation needed (Unless you go buy it from steam.) Although it might still be a good idea to try and get a digital download version. The original 4 CD install method is a pain in the arse.
Civ 4 or 5, some of the old X-COM games can be fun. Any of the Total War series and yea +1 to Minecraft good way to kill some time.
I was going to specifically put it, but then I thought Baulders Gate 2 was longer and more of a classic, and then just went for "most other RPGs" which KOTOR would have come under, but yea +1 for knights of the old republic
I would recommend folowing games for offline-play: Revenge of the titans (nice towerdefense game) World of goo (kinda puzzle game, i thinke) Machinarium (if you like some point-and-click) Defcon (nuke the earth) But i have to admit that civ is an awesome game series.
Any of the Elder scrolls from III onward are great. Minecraft Torchlight Generally I think RPGs are the best offline games, shooters tend to get boring after a couple of playthroughs because there isn't enough variation.
You can play Fallout 3 this game kept me busy for a long time, also Dead Space and Company of Heroes.
I'd also recommend the KOTOR games. Epic storyline and gameplay, some of my favourite games of all time