This year Robert and I have been doing unusual mind puzzles. We started with just riddles, but now we have moved to the Rebus puzzles or in other words the Word Picture Puzzles.
answer: 3 blind mice (there are no I's)
These are simple puzzles that I drop into his lunch and then the next day I put the answer on the back of the new puzzle. Towards the end of last year he had several friends helping him figure out some of the puzzles (including the lunch lady.) These are so simple to print out and then cut into slips and drop in.
Here are 2 websites that I turn to for these puzzles and other fun ideas.
http://kids.niehs.nih.gov (go to the Brainteaser and puzzle link
another great site
http://www.brownielocks.com once on this page I go to the riddles and jokes link.