Okay, Aunt PJW - six months is quite a short amount of time to essentially plan everything in a wedding by yourself, so take a deep, relaxing, cleansing breath right now. Haha.
Knowing exactly what era she's going for would be very helpful. When some people say "vintage", they think 1960s, and some think 1920s. Also, ask her if she has a color scheme in mind. That's very important!
The favors could be small bags with candies in them that represent that era - if she's going for early 20th century, since it's at a beach, little individually wrapped pieces of salt water taffy would be very cute.
I strongly recommend sending a save the date before the invitations because it's so close to the wedding day - since you are on a tight financial and time budget, I would strongly recommend sending an electronic save the date. A really cool trend nowadays is to have the engaged couple film a video, announcing the big day, and post it to YouTube. Free, fast, and fun!
Make the invitations simple! Find a really nice script font online and match it with a simple font (I like Century Gothic which is already in Windows as a font), and alternate between the script for their names and the simpler font for the details. Make sure your printer has fresh, new ink in it, and print them yourself. Most craft stores have invitation kits already, which range from super frilly to very basic.
For the food - hors d'oeuvres! Make it a cocktail-style reception with small snack foods - make sure you incorporate their favorite foods in to it. I have a bride now that wants mini peanut butter and jelly sandwiches cut with a fancy cookie cutter - very personal and cute.
Good luck with everything! And remember what I said before - breathe!