For decor you can make a center peice out of legos that spell out his name.
You can go to the dollar store and get small little lego sets for a dollar, do that plus lego candys as favors (also at the dollar store) the candy taste like crap but the kids wont care.
for the cake you can use these candles
or just purchase a rectangle cake and place little lego men on it, make them look like they are celebrating his birthday, or bowing to the candles, you can even get a fireman set for on the cake make it look like they are trying to put out the fire, that way your son will help save lego town when he makes a wish!!
Here is a cute idea for invites
at 1:09 AM on Aug. 17, 2010