Thank you for all the suggestions. I've checked out some reviews of the games you suggested, and I think there are several that my Dad would like but I decided to go for World of Goo, and maybe Ticket to Ride (heck, it's just seven bucks a pop!). However, after reading all those reviews I also now want an iPad. :-D

Contre Jour HD is on sale today (nov 11) for $1. It's very much in the spirit of World of Goo.

Cut the Rope


Runners up:

Sid Meier's Pirates!
WordJong
Peggle
Sim City Deluxe
Civilization Revolution
Game Dev Story

Definitely Neuroshima HEX!. It is iPad version of very popular board game in Poland (same name), which takes place in Neuroshima world (popular rpg game). It has very clear rules, easy to learn after 1-2 plays. You can play it with friends (up to 4 people on one iPad), or with AI (4 levels of difficulty). It is optimised for touch interface, and plays very naturally. I have the board game, and it plays exactly the same way as iPad version.

For me, it's one of the best games available on iPad. Oh, don't look on youtube videos, I couldn't find any with actual version of the game (which is optimized for iPad).