I would do a signature drink for the shower either alcoholic or non. Find a great pink punch or something similar and call it a Pink Cadillac! Pink/white/silver plates and napkins would be inexpensive and then you could spend $ on the fun stuff like favors and decorations. I found these for favors http://www.wrapwithus.com/Products/Retro-pink-cadillac-we...
centerpieces? http://cgi.ebay.com/XL-PINK-CADILLAC-50S-PARTY-BALLOON-FA...
Since their a little bit expensive I'd do maybe every other table along with some pink/silver balloons and then on the alternating tables just pink/silver/white balloons.
I found lots of fun decorations like records when I Googled pink Cadillac favors. You can even get a poodle skirt to wear (or to use on the buffet table maybe around salad bowls or punch bowl)! How cute is that!? Maybe use vinyl records in the centerpieces or hang them from the buffet and/or gift table. Play 50's music. If the baby's room will be pink Cadillac everyone could decorate a vinyl record to hang in baby's room using neon/silver markers or paints. If you want to play games you could have a 50's music lyric fill-in-the-blank or karaoke if the attendees are a fun group. Don't forget about Jukeboxes. You could use pictures of them in your decorating (I bet Party City or Oriental Trading would have them) or to even have a Jukebox there would be awesome but maybe out of your budget.
I could go on for days. I love theme parties! Let us know what you decide or post pics! Good luck and have fun!