We have stayed in sea pines for the last several years each summer. We always trek all around, and visit the preserve too. I'll trade you info in exchange for
DVC HH info since we've never been there and now are thinking of exchanging that for our regular vacation!
We have always had a visitor pass because we're renting in Sea Pines, but I do not think they're strict - you can tell them if they ask that you are looking around, that's reason enough. Or the forest preserve or whatever - they really just want the $5

. I don't think you buy the passes at the guardhouse, though. There's a visitor's center right to the right in an office-looking complex just before the gatehouse - there are signs.
The forest preserve is just on your left as you come in. If you drive further you'll see Lawton Stables. The preserve is beautiful, especially if you're not from that area since the plantlife is really different (we're in Ohio) and we've found lots of neat birds and even the occassional alligator. Actually, every year we go "alligator hunting" all over sea pines on our bikes and keep track of how many alligators and turtles we find on each trip. You can ride bikes through the front section, but the rest is walking trails only, and a stroller really doesn't work great back there if that's a consideration. The only thing I will say is go before dark and bring your bug spray or you will be eaten alive - it's the one really unpleasant thing about the preserve.
I would definitely call ahead on the stables, and they have pony rides as well if you have little kids. We've also gotten sugar cubes from the grocery and you can feed them to the horses right along the bike trails.
If you have kids you should try to catch Greg Russell at Harbor Town - he's free and there's a great playground there, but parking is very tricky. You can also park just after the gatehouse in Sea Pines and take the trolley to Harbor Town.
So that's a really long post, but if you have any more questions, I'm happy to help.