It is satifying to work with a spreadsheet because it does such a nice job of formatting the work. This gives the student a feeling of accomplishment.

Computing some sort of average having to do with sports would be interesting for many 8 year olds. This helps cross a language barrier, since sports have a well-defined vocabulary.

Another good exercise is to have them build a computer. I was able to get 30 students to make their own slide rules in twenty minutes. I gave them a quick demo of an adder by using two rulers. Then I gave each of them a log table (with the numbers expressed as lengths) and a ruler, and they made slide rules out of paper.

Another example was a prime number generator. One group wrote the sequence of numbers on small slips of paper and handed it off to a chain of others who implemented the sieve of Eratosthenes. It was pandemonium, and fun to watch! (Hint - the bottleneck is at the divide-by-7 table).

The trick to all of these exercises is prepare by practicing.