Party blowers and confetti are a fun and cheap addition to any party. Also, transforming a space with a bunch of giant paper globes, starbursts and streamers can make a big impact for a small investment.
When you send out invitations, try to add some kind of three dimensional object to it. A temporary tattoo they can sport coming to the party, a wand or fun buttons. It helps bring the party to life in their minds and makes it feel real and a little bit special.
Scene setter kits are affordable, and fill up one wall space creating a powerful emphasis without taking up any space.
A carnival is one idea that really is adjustable to the group. Carnival games can double as décor, and décor can be as simple or fancy as you wish. Also carnival foods come in a variety of the simple like cotton candy, hot dogs and French fries, to the more complex like funnel cakes and sliders.