Go to garage sales and get a couple old couches, chairs, and bean bag chairs and you can put slip covers on them. Get some hanging Christmas lights (white, blue or white&blue) and string them up around the inside of the shed. A really big, comfy rug. Does it have electricity? A radio. Posters of your guys favorite movies, bands, and actors, too. A couple stuffed animals. A small coffee table so you guys can eat and drink out there. Big, fat, squishy pillows.

Thats about all I can think of.