Lettuce is easy to grow from seed. It grows best in the cooler weather in spring and fall and will “bolt” in the hottest months and taste bitter (not sure if rabbits will still eat it, but people generally don’t like it after bolting). Spinach is similarly easy to grow and also likes the cooler weather.
All of the herbs can be grown in pots starting from seed as well. Some will live over winter (thyme often does) and some only like hot weather (like basil) so you can grow a variety that will do well at different times. Parsley and cilantro are especially easy.
I agree with Beka though that it would be extremely hard to grow enough to feed a bunny without a lot of land. My pair go through 5 large heads of romaine and 4 bunches of parsley a week. Even with a couple 4×8 beds in the garden I can’t keep up with that demand. It is definitely fun to give home grown veggies as a treat though! I especially like planting herbs because I use them in cooking all the time. Instead of buying a huge batch at the store, I can rip a bit off of my plants and give some to the bunnies while putting some in my dinner.