Write the invitation in contrasting colors on it. Deflate the balloons fully. And mail them. Have colorful streamers, party poppers, masks and paper-hats in plenty. Hang colorful paper ribbons across the walls or the ceiling.
Giant props are always fun.
For the same reason, when things are ten times bigger than they should be, people derive the fun from them. Glue various trinkets in abundance across the room and on the dinner or buffet tables. Catch up the Carnival parade mood. Hang them each by a string wherever you like. Also have balloons - purple, green and gold.