Find a beach. This may seem obvious, but there are people in Minnesota who will need to find a sandy spot on the shores of a lake. Clearly, the chances of taking your gal to a real beach will be vastly improved if you live on the east coast, the west coast, or the Texas panhandle. (And no, buying a kiddie pool and filling your den with sand doesn't qualify as "the beach.")
Dress appropriately. The point of a beach date isn't to catch as many rays as possible and get an even tan; it's to hide your physical defects without being too obvious about it. If you're pigeon-chested, wear a slightly oversized T shirt, and if you've got a prominent beer gut, wear a very oversized T shirt. There's not much you can do for your spindly legs, except to find a pair of shorts that come down almost to your ankles.
Perfect your "I'm with her!" look. The reason your girlfriend wants to go to the beach is because she genuinely likes the sea and sand; the reason you want to go to the beach is to show off your girlfriend to all the other guys (of course, this only works if your girlfriend is showoffable-tell her to work out a bit before you commit to a weekend). Just stay nearby, in case that sand-kicking bully from high school tries to steal her away.
Build a sandcastle. Women love providers, and nothing screams power and reliability like a guy who can build a five-story, terraced sandcastle with a three-car garage and breakfast nook. The only trouble is, it's hard to devote the time to this without leaving your girlfriend dangerously unsupervised. Try to round up some kids as slave labor-they won't know they're being taken advantage of.
Run in slow motion. Remember how cool David Hasselhoff looked when he sprinted across the beach in slo-mo, a confident smirk on his face and a surfboard under his arm? No woman can resist a man who's perfected his slow motion when everyone else is moving in real time. The only trouble is, your mojo will be stolen by any Pam Anderson lookalikes who happen to be slo-moing nearby.