Super Meatboy, Downwell, and Crypt of the Necrodancer are all games that are fun and can be played 5 minutes or 5 hours straight. Awesome buys.
That being said, a game that fits the leveling and spec style game you like would definitely be Risk of Rain. Such a sick game, cheap and again can be played for a little or a while. They just game out with Risk of Rain 2 on early access as well. Looks solid but they have a lot of big things coming before it's full and I'd wait for that. The first one is great though.