For my son's 6th birthday we decided to have a Lego party. For the invites we downloaded two different types of Lego fonts - one that wrote in Lego bricks & one that drew Lego men. I cut up several Lego boxes that we had & pasted the characters on the front of the invite.  Inside was the child's name in Lego block font. The main message was in a standard blocky font Are you a blockhead? Have you Technic skills? Come for a Mindstorm be a Bionicle Commando. Lego and be an adventurer at Daniel's Lego party. Then put contact details & finished with "(some assembly required)" and a row of the Lego men font.  The night before the party we sent our son away for a sleepover as we had some work to do... We cleared out our sunroom hired a balloon pump and started blowing up 750 balloons to bury the room 1 metre deep. (we took a photo of the room every 50 balloons - it made for some great scrapbook pages afterwards). As we blew the balloons we broke up a a small Lego model for each of the 6 boys attending - one block at a time into a balloon of a colour for that boy - ie all of one model went one block at a time in a blue balloons another model in red balloons etc. We also got 6 large boxes and decorated them like giant Lego blocks - one in each colour of the balloons with the blocks in. To keep the balloons in the room just before we left we put a childs stair gate over the inside of the doorway & shut the door.   When the boys arrived they were given a "Agent" card with their colour Lego man on it.  From then they were only supposed to use their code names... Agent blue Agent red Agent white etc.  They were then given a box of Lego and told to work together to contruct a huge radio receiver tower out of the Lego & when it was high enough a message could be received by headquarters. When the tower was about a metre high we played a pre-recorded message telling them that the Evil Brickster had stollen all the Lego plans and for the fun of children everywhere they would have to follow the clues to hunt him down. It would pay to gather equipment on the way.  The clues then sent them on a scavenger hunt collecting things like handcufs a ladder a crowbar rope on the way.  The final stop was outside beside the room with the balloons in. The boys climbed up the ladder and through the window into the balloons that came up to chest height on most of them. The looks on their faces was priceless. We then told them they had to find the balloons of their colour and when they found one with a peice in (by rattling the balloon) they were to burst it inside their giant Lego box/block. They had so much fun - it took them a while to work out if they popped EVERY balloon of their colour they didn't pick the same balloon up 5 or 6 times. When about half the balloons had been popped & most of the blocks found we gave them party food & cake (while a couple of adults looked for the last few missing peices).  The cake was a rectangle one with 6 mini muffin tin cakes on top to be a Lego block.  The whole thing was iced bright blue & surrounded by Duplo blocks. After the food they sat down & assembled their model - from the plans (also in balloons) that had been rescued from the Brickster.  There were only 4 blocks missing & we just made them up from Lego that we had. Each child when home with as many balloons as their Mum & Dad would let them have and the small Lego model they had got from their balloons. This was a party that our son would never forget."