Plan on spending most of your day at the beach. The beach house does have a pool and lounge if you need a break from the sun, as well as a small arcade, pool, Foosball, and ping pong. Back at the resort, there's another pool with a water slide, horseshoes, basketball, shuffleboard, a marshy area, places to sit and relax, etc.
For food, we have two places we always hit. The first is the Crazy Crab. There's actually two, one over on the North end of the island and one over on the south end of the island. You'll have to pay parking if you go to the one in Harbor town (south) so hit the one on the North end. It looks like a dump but the seafood is fantastic. Here's a
link to their website.
Then there's Old Oyster Factory. They've got excellent seafood as well and a great view. Be warned, they are very popular, so you'll want to make a reservation ahead of time. Here's a
link to their website. If you can only do one, I'd say do Crazy Crab.
We love Hilton Head. We've done it a bunch of times for our June vacation and we go there every December during winter break to get a break from the chilly weather and relax (the concept of a staycation is beyond my father). It's gorgeous and a ton of fun. I highly recommend it. In fact, writing this is making me miss it!
EDIT: No, Hilton Head Diner is not that great. The resort does have a quick service place for breakfast that's pretty good. You can hit there and eat outside while looking over the marsh.
