My father & sister own a Marriott timeshare in Hilton Head since 1993 and we absolutely love to go. I believe we have not gone only 3 times, when they exchanged to go to other locations; every other year we choose HH. The timeshare is oceanfront and the Marriott property is top notch. We get a free pass to Sea Pines plantation since the resort is just outside Sea Pines' south entrance.
We never plan any activities... only meals! It's funny, but we cook almost all our meals, so we come prepared with our groceries, plus we make several grocery stops at the Bi-Lo (love it!).
Anyway, the only thing we do plan (besides meals) is to rent the bikes. One of my favorite activities is to ride the bikes on the beach and Sea Pines. HH's bike paths are amazing. I also love to visit the lighthouse.
One recommendation: visit The Chocolate Canopy. If you like chocolate, you will find your heaven in this place. It's a mom & pop shop that makes their own chocolate and it can't be beat. They are famous in the island for the high quality chocolates.
Other than that, we love to hang out at the beach or the pool. Maybe go to the outlet stores; but really, I just go to relax. My DD7 loves it there too; she loves the pool.
I know my experience in HH is not Disney (yet, now that I bought points I might book there one day), but I've been going for 10 years and I never get tired of it. It's a lovely place.