Although it's cold & rainy (or snowy in many parts of the country), you can explore Farmers Markets for interesting veggies for your rabbit. In Sacramento, there are great deals on herbs (for treats), and veggies you just can't get in the regular markets. You can find a bundle of small carrots with tops (the best part) for only $1.50, a large bunch of arugula for $1, or a huge bundle of parsley for only $1. Unusual veggies also can appear. I am waiting for brocolli greens to return. I found these last winter and spring. They look similar to collard greens, but much larger & sweeter tasting. A very large bundle was only $1.
Be sure to test any new food for your rabbit by giving a small amount & observing any adverse reactions.
Food at Farmers Markets are often (but not always) certified organic so you don't need to worry about your rabbit's food being exposed to artificial pesticides or herbicides.