I promised pictures. Here they are!!
The invitations: We found a printout of a legohead online, printed it out and used it as a template on yellow card to make these. Then The Boy drew the faces on.
At the party, we first held a building competition. We bought 1 car for each child, then drew numbers out of a hat so each child got a random one. They got to keep the cars. After that, we guessed how many lego pieces were in the glass. Correct answer? 63. Closest answer? 44.
Then, using the cars the children had just built, we held races!! This was extremely popular.
When that (finally) got boring, we threw water balloons at two little legomen in the carport, trying to knock them over.
It seemed to be a success. Everybody went home happy and tired!! And the Birthday boy is happy, too.