I can't believe I've never written a review of @Peace Cafe, as I've been going there for years. This simple but charming cafe is in the Garden City community, about fifteen minutes into the mountains above Xindian. Since its opening a few years ago the menu has simplified somewhat, but they still offer delicious, healthy meals based on seasonally available organic foods, at very reasonable prices. It's a nice escape from Taipei Air.
If you are visiting Xindian (for Bitan Lake or hiking in the nearby mountains) it's well worth the trip up for lunch or dinner, but it's best to call first to check they are open. Consider having lunch here and dinner at the Green Pool Loving Hut (by Xindian Station) or vice versa.
If going to Wulai (which I don't recommend) it's essential to take supplies as there's nothing vegan (or safely vegetarian) available there. Consider having lunch at @Peace Cafe, then walking or taking the bus down to the main road to Wulai, and then perhaps finishing with dinner at the Green Pool LH before returning to Taipei.