I give my own pair Oxbow Simple Rewards treats daily, as they’ve got a good fiber content and my buns love the apple/banana flavor (and, it seems, the bell pepper flavor too). They get Oxbow pellets for breakfast every day, a total of 1/4 cup (1/8 cup per bunny, they’re small ones); I use Essentials since it’s very high-fiber in content. I was investigating their Garden Select for a while, but it’s more expensive than Essentials and actually slightly less fiber. I’ve experimented with sprigs of herbs in the past, but while Panda likes them, Fernando has generally turned up his nose at them (he likes them in salads, though) so I don’t want him to get left out.
Another treat to be very careful about is oats (plain rolled oats). I was giving them to Panda and Fernando for a few months based on the recommendation of another local bunny owner, but I found out at their last biannual exam they were gaining too much weight so I stopped.