You can drive up fairly close to one of the walls. Fort Rock State Park is a day-use park. There are hiking trails all through the area. We were on our way to dinner, so we didn't have a lot of time to spend here.
In the town of Fort Rock there is a museum where they have a collection of old buildings that they've brought in. On a previous visit we took time to go to the museum. It's well worth a visit. You also have to check out the tavern there in town - not the fancy, new restaurant but the local tavern. It's also worth a visit.
From there we headed off to dinner another 20 or so miles down the highway. That dinner deserves its own blog entry!