First off, burgers need not all be beef. Turkey burgers are also delicious, and they taste pretty similar. Salmon burgers are absolutely wonderful, though they taste completely different (obviously, they taste like fish). If you get salmon, make sure that it is wild (more sustainable), preferably Alaskan (less mercury). Chicken breasts make an excellent sandwich as well. Finally, if there are any vegetarians in attendance, garden burgers are pretty tasty. Many people grill whole portobello mushrooms like burger patties, and while this is not to my taste, they seem to love it.
A Delicious Looking Salmon Burger
Image taken from robtrent's Flickr photostream under CC license.
A portobello mushroom burger
Image taken from karitsu's Flickr photostream under CC licence
Frankfurters are a must for most grillers, but consider replacing a few dogs with a flavored chicken sausage or two. There are tons of varieties, meeting the needs of every palette: sweet, smokey, spicy, etc. Maybe chicken bratwurst with sauerkraut and stone ground mustard catches your taste buds, or chicken apple sausage with diced onion and Dijon. Even if neither of those sounds like your style, I guarantee there is a chicken sausage that will suite your palette.
If you are feeling really adventurous, check out my whole brined chicken recipe. Less, but still a little adventurous? I did the exact same recipe with a pack of chicken thighs to great effect recently. It is much easier to manage on the grill than a whole bird.
And remember, it doesn't have to be all about the meat. If you only grill burgers and dogs, people will eat more of them, but if you also provide some crowd pleasing vegetables like corn on the cob, you don't have to provide as much meat. That should save you a few bucks too. See link at the beginning of this post for a few more ideas.
Finally a note on fuel. Propane is vastly more efficient than burning coal, both in production and end use. Of course I am a huge hypocrite because I love the smokey flavor of real charcoal, and don't plan on replacing my little Webber for some time. I am told you can get a similar smokey effect with liquid smoke, but I cannot vouch for it myself (I haven't tested it because I don't have a propane grill), and don't really know how to use it (does it go in the marinade? added at the end? comment if you know).
If you just happen to be in the market for a new BBQ, and also just happen to have a lot of disposable income, check out the Big Green Egg. Although they are very expensive, they are very, very well reviewed. The design allows for extremely fuel efficient cooking; many people report using one tenth the coal as they would in a conventional metal 'Q. If you figure out how to find one used, let me know.
A 'Big Green Egg' BBQ and smoker
Image taken from DerekL1963's photostream under CC licenseThat is it for now, enjoy the food friends and family on your fourth.