I have just recently done a lego party for my son in dec (8yrs).
1st game was to build lego racing cars to then race down a ramp we made out of a slide !!!
treasure hunt finding the clues, each clue had a lego piece when put together formed the answer to where the treasure was burried.
Made a treasure box and filled it with chocolate coins and lego pirate stuff.
pick up lego with chopsticks, weigh, winner gets lego stationary.
guess how many bricks in jar
decorated the house with blue,red,yellow green streamers and the table was decorated with hellium ballons in lego colours
I made lego pots to put on table.
invites hand made red card with lego images from website.
blue party bags with lego font for names.
Lego keyrings, lego colour page then the prizes from games.
I made a lego cake which went down very well
I found loads of lego stuff from amazon and lego.com
google lego party ideas in images
Good luck, its great fun organising it, i will post some pics in my profile