I can’t speak highly enough of Oregon Caves. A recent article in the Mail Tribune newspaper highlighted that the monument, along with Crater Lake, are among visitors’ favorites.
I gather that if you’re driving inland to the caves, you’ll be taking Interstate 5 north. If that’s the case, consider a 10-minute detour south from Grants Pass to stay at Valley of the Rogue State park, one of Oregon’s premiere campgrounds. It’s right on the banks of the Rogue River and has extensive and very well maintained facilities.
If you were visiting in summer, I would suggest continuing on to Crater Lake, Oregon’s only national park, but snow can make the park difficult to access this time of year. If you’re up for a snow day, though, the park rangers offer free snowshoe hikes on the weekends and provide the snowshoes.
If you decide to travel that way, take Highway 97 to central Oregon then Highway 26 through the Warm Springs Indian Reservation and Mount Hood National Forest to Portland.