I've done loads!! As an ex-rugby chick, dress codes were enforced weekly - so....
Madonna (80's look) string vest, fishnet gloves, leggings etc
70's - Love train! Charity shop fave!!! Big flares, platforms, flowery tops and I have the biggest blonde afro EVER!!
Old Granny - one of my faves! Old granny dress from charity shop, hair in rollers and big bottle bottom specs
Also done Nun, Cowgirl, Smurf (ha), toga (easy!), Lara Croft (not fun unless you're comfy showing your bod!), PJ party, 9 Carrot gold (gold outfit with 9 carrots round your neck!), Bierkeller girl - including wired pigtails to make my hair flick up!!
Ebay is great and charity shops too - so doesn't have to be expensive.
Obviously, if you have money to spend, hit the hire shops - they have some great ones.
Let us know what you decide - enjoy x