2in trailer hitch. The Forester should have one, and it's by far your best option. Aftermarket hitches should run ~$200 installed (by U-haul or potentially Subaru). The hitch itself is ~$130 if you're comfortable working on cars. The installation is relatively simple, on par with changing your oil. The frame rails will be pre-drilled and tapped for bolts, so it's a matter of bolting it up and torquing it down.

With a hitch, there are a wide variety of bike rack types. Simple top tube racks are the cheapest, and work well for standard diamond frame bikes. Wheel tray type racks are more expensive, but work far better for full suspension mountain bike where top tube mounting is problematic.