We had loads of fun (and learning :-) making chain reactions yesterday afternoon. We used blocks and dominoes, but the children especially enjoyed "stick bombs". This experiment involves weaving craft or ice block sticks together to create tension (potential energy), so that the sticks fly up when released. Here is a link in case you want to try this at home with your child over the long weekend.
Hint: I couldn't do this with ice block-sized sticks. You need the larger ones, tongue depressor-sized. You can get these at $2 shops or hardware stores. Have fun!