The best part is that Couch Potatoes drop from most enemies in Grizzlehiem. But I usally farm the Forest Pests in Saavasrad Pass (sorry if I spelled it wrong). I usually get about 1 Couch Potato every 20 min of farming.
I suggest fighting those guys as they only have 395 health.
The Couch Potatoes is a rank 3 plant that will drop 1 high XP mega snack at elder. It would only need water and music and it only gets rank 1 pests. I would prefer for you to cast a pest prevention spell. That will guarantee save you on energy every time you take care of your plant's needs (except for when the prevention spell expires of course lol).
Here's a (short) list of all the Mega-Snacks it could drop at Elder: Fancy Yogurt - 50 XP Captain Cantaloupe - 46 XP Golden Wheat Bread - 40 XP
It requires a Large Plot and will cost 1,500 crowns each if you buy them from the Crown Shop but you can also farm for them. As for which enemy to farm at for the plant, I would say the fire hog guys in Boar camp in Savarstaad Pass in Grizzleheim or the Ice bear guys in Savarstaad Pass but they are closer to the teleporters and are right up the road to the left as soon as you enter Savarstaad Pass. I hope this helps