My son turned 7 this year and wanted to have a lego party. We went to Legoland last year and had a picture of him in front of the sign.  I used that for the invitation and wrote, come help Zack build some fun at his 7th party!   For the cake, I made minature cakes (used mini loafs) and spelled out the word lego (you could do brownies too). We frosted each on of them a different color to represent the colors of the legos. I used reeses peanut butter cups for the top part (put them upside down). I used white almond bark and died it to match each of the colors for the cake.  For decorations, I had balloons, streamers and banners the color of the legos.  I could buy them at a discount store instead of spending the extra money at the party stores. We also used all of the legos that he has created as part of the decorations throughout the house (we had about 15 of them around the house).  He could talk to his friends about how fun it was or how hard it was to make.   For activities, we bought legos to make cars to race and had my son build samples of them for his friends to look at and get ideas on how to do it.  We provided the wheels, frames, one driver and extra pieces. You can buy the kits for the wheels and frames and extra people at the lego store.    The kids built their cars and then had races on a ramp that my husband built for racing (like the one at Legoland). My husband made them out of plywood and separated each one by a smaller piece of trim so they would stay on their own track down the ramp. They had a blast with these.   I think some of the parents liked it too.  They were getting into making them as well. We also had the contest of counting legos in the jar and building the highest tower. We had a lot of kids there so we split them in teams for this activity. The team that won, got to pick out of a grab bag of different toys.  For goodie bags, we found colored bags and put in lego candy (from candy store), lego fruit snacks and they got to take home their cars that they created.  This was an awesome party and the kids loved it.