Gonna be traveling for 3 weeks and need something to keep me distracted....

You can get addicted to real racing 3...zombie highway and zombie gunship are cool

Candy Crush?

Zombie trailer park, drop 7, flow, 8ball pool

Kingdom Rush and Kingdom Rush Frontiers have killed a lot of travel time for me.

All of the sky gambler games are exceptional.  They are third or first person flying games where you control your plane by moving your ipad, and have very responsive controls, realistic planes (I think they have a ww1, ww2 and cold war era versions) and good graphics.  Very arcady though, don't expect a deep flight sim.

Infinity blade one and two are very good.  They are fighting games where you attack and dodge by screen swipes.  A very good action game that is perfect for touch screens.

My favorite game ever on the ipad is the room.  it is an atmospheric puzzle game.  Not easy, but not too challenging.  They made a sequal but I haven't played it yet.

There are also some remakes of old games on ipad, but they are hit and miss because you are talking about porting a game that used a controller or keyboard to a touchscreen.  The ones that I like are mega man x, wolfenstein 3d, and doom.  The best one is probably knights of the old republic, which works VERY well on a touch screen, since the combat lets you pause and issue commands.

"World of goo" is a fun physics based game. If you are into stuff like that.

Kingdom Rush and its sequel,

Tiny Thief

Oceanhorn

Where's my water 2 is pretty fun.

oceanhorn is a decent legend-of-zelda-esque game.  Not free though

Modern War

Bump for more ideas

Hill climb racing.