The 1Up racks are pretty much the best rack that I have ever hitched to the back of my car, I can't say enough good things about it. An option for the 1Up are the wheel locks that lock your wheels into the frame. While this will keep your bike protected, I always worried about someone coming along, undoing the skewers, and swiping the rest of the bike. Then I came up with a nice solution that is pretty straightforward.
The beauty of the 1Up is that they have wheel locks that lock out your wheelset, along with rack locks (at the hitch level) and tray locks. Should you buy all of the locks at the same time from 1Up, they will key them all together so one key rules them all. But one of the down sides is that the wheel locks only truly protect your wheels, not the frame and other components (which are more expensive than your wheels in most cases.)
I realized that if you take a regular cable and thread the lock through the eye of the cable, run it through the middle of the frame, then thread the other lock through the other eye, it holds it in place. This protects the frame and adds an extra level of protection. If I was leaving the bike overnight, I would probably use a much heavier lock, but for a few hours at a pub post-ride it works fine.
Someone else on BikeMojo also recommended a lock on Amazon, this looks like another good option:
This looks like a nice clean solution, locking right to the hitch. You can buy it on Amazon here.