I've been doing a lot of context switching between python and ruby lately. I've found it increasingly hard to go between the two just from language differences alone. These "kata" are some quicky tasks I'm slowly building up to practice each day. I don't do it every day but if I'm having trouble focusing, I grab one of these and hack on it for a little bit to loosen the cobwebs.
General rules apply. Throw away the code at the end and such. It's just a fun set of exercises. Some will take longer than others. Use whatever works best for the amount of time you have free.
If you have ideas for others, I'd love to see them. Feel free to fork or watch. I'll just add random ones as I come up with more or they're recommended.