Divide your children up into groups and give each group equal amounts of blocks: (ie. wood blocks, Lincoln Logs, Legos, jumbo blocks), Styrofoam or plastic cups.
The groups will race against each other to see who can build the tallest tower, in a given amount of time.
After time runs out, measure the towers to see which group has built the tallest tower. Then mix up the groups and play again.
You might enjoy these cute sponge blocks courtesy of Toddler Approved.
You can even play a simple, but fun game of Jenga.