One good way to kill some time and exercise your brain is by solving puzzles in Interactive Fiction.
One of the more difficult IF games is, "Max Blaster and Doris de Lightning Against the Parrot Creatures of Venus" by Dan Shiovitz and Emily Short. It was selected as the winner for the Spring Thing 2003. One of the coolest things about it is being able to switch between characters when they are together. You can download it from the IF Archive along with some helpful hints. You'll need a TAD3 player to read it. Some of the puzzles can get frustrating. If you get stuck, you can always refer to the hints. I found some areas where Doris is easier and others where Max is stronger. One good way to get unstuck is to restore back to a point when they are together and switch to the other character before proceeding. There are a few glitches but the entire thing is playable to the end. It took me about 7 hours to finish.
Another good Sci-Fi IF game is "The Elysium Enigma" by Eric Eve. It won the prestigious XYZZY award for best game in 2006. There is a walkthrough but I suggest trying it on your own first.
If text-based games aren't really your thing you might try Samorost 1 and Samorost 2. These are point and click flash games. Samorost2 is shareware but it's worth registering.