You could do what DeanandSam said, that sounds cool. And if you have some money you could serve all your drinks in a Supernatural Cakehole travel mug and you could have Jensen and Jared jump out of a cake but you could also just make a boring old SPN cake ;)
There's plenty of things you could do.
Happy birthday btw :)
IF you hit a party store,
pick up tombstones, skelton, for outdoor dec's,
hang meadowsweet (similar) above the door,
Silk screen t-shirts with the tatoo above the "heart"
or print off stickers of the tatoo to put on one's body,
Paint some of the Spell "markings" & hang them around the room (check out Hell House episode),
have a game of Jeopardy (or such) of answers of the show for which have to be answered in the form of a question,(with a prize of course),
make the tablecloth
(a black cloth, painted white/silver or gold? devil's trap)
that is used in the Seance episode of Pamela's,
could go on, but you get the drift.
If this is too late, save for another time or just have a Supernatural party~