My son has really enjoyed doing worksheets. Since he's not in school yet so he thinks they are fun. Little does he know that it's homework.
Why not put together a fun activity book of your own? What about a binder of worksheets that you can keep in the car or take on an airplane? Decorate the binder and make it personalized and it is easy to refill once the activities are finished.
Here are a few websites that I've found that have TONS of activities for ALL ages for free. Puzzles, Sudoku, matching, spelling, coloring pages, tracing, math, and dot-to-dots.
I also found this great idea here for a Sunday activity book to take to church to keep the kids busy. She printed the Funstuff pages out of the Friend Magazine found here.