MamaBunny,
when I started adding fresh foods, I made sure to feed my rabbits "dry matter" first to take the edge off their hunger. I then introduced very small amounts of fresh foods- for a Nethie I would probably start with a 1" x 1" piece of greens, and give slightly bigger pieces each day. You could probably triple or quadruple that with no issues, but I am a bit of a "Mother Hen", and err on the side of caution.
Feed the same food item in increasing amounts for 3-4 days, then introduce a small bit of another fresh food along with it, and watch for adverse reactions.
Once you have a doe acclimated to fresh forage, provide the same feeds when her kits start coming out of the nest, and they will develop the correct gut flora to process it because they only nibble small amounts at first and have also been ingesting the doe's "cecal pellets" which have the microbes needed for digestion.
Feed carrots, apple, celery, and winter squash sparingly. The bulk of fresh foods should be dark leafy greens.
The link that OAF provided will give you a good idea of safe weeds and other items that you might not normally consider to be "food" that are healthy forage for rabbits.