I started mine with romaine lettuce, then a few days later I bought some carrots with LONG tops on them (I eat the carrots/they get the tops) and some turnip greens. all three have been a hit.
it's best to introduce veggies one at a time (some bunns may not tolerate certain veggies well - this way you'll know which veggie is causing the problem). you can introduce a new one every few days. start with smallish amounts when you first introduce a veggie, then you can start feeding larger portions once you confirm that they're tolerating it well.
oh, and prepare yourself for a lot of "awh!!" moments as you watch them nom on veggies - it's crazy cute, especially when they're 12 week old babies
House Rabbit Society (http://www.rabbit.org), a reliable source of bunny info, says you can start gradually introducing veggies at 12 weeks... and my vet agrees
Thanks for all the info...I need something to get back on his good side lol. We had a very small grooming session tonight. He was seriously ticked off at me afterwards