My last pair of boars had two bendy log bridges/tunnels (which Comet loved eating, for some reason), two seagrass tunnels (which Comet loved eating, for some reason - I'm sensing a pattern here) and 4 carrot cottages that they stripped all the good bits off and left the frame behind, which was awfully sensible of them. Comet only really used the carrot cottages, Blitzen used all of them at one point or another.
Whatever you get for them, you have to be prepared that they'll try to eat it - like Comet did. And Jake once ate an entire plastic bowl and nearly gave me a heart attack in the process.