I have a lot less time for games these days then I used to. Where once I could devote entire evenings to mapping out dungeons and patiently questing away, I now find that I fall into the 'casual gamer' slot. Casual games are those you can pick up and play for a few minutes at a time, and then put down. They don't require a big time commitment, and can be good for stress relief.
Right now I'm playing N+: The way of the ninja. I first came across it as a flash game on a web page a long time ago, but it's still around. In fact you can download the full game for free from their web page, in either Mac or PC flavors.
I bought a copy for my Nintendo DS Lite so that I could play it anywhere around the house, and I wasn't disappointed. It has a huge number of levels, and the levels for each version of the game is different. (It is also available on the PSP or Xbox360 if you have one.)
The plot is simple: You are a ninja. Grab all the gold you can and get to the end of the level before the time runs out or you die. This can involve navigating mazes by running and jumping, and avoiding traps and enemies. There are no health bars or power-ups - if you touch anything dangerous you die. There's no penalty for death, aside from then having to restart the level you are in. To add to the challenge, you have to complete 5 levels in a row before you can save your progress. You have no weapons, and you can only run and jump, with the added ability to cling to walls to a degree. It's a 'timing and reflexes' challenge.
Give the free version a try, it's really entertaining. But be warned, the difficulty curve is a wall, and it's literally the hardest game I've ever played.