I have been a couple of times and have called Hoop Dee Doo directly a few days before my ADR. I've always received a few options, usually pasta w/ marinara sauce or a choice of grilled or steamed veggies w/baked potato. I think I've also read of someone who was able to get a veggie burger.
I was quite surprised - I chose the grilled veggies, and it actually turned out to be one of my favorite meals of the trip! The portion was plentiful, and while simple, it was perfectly seasoned. It comes served w/ a baked potato, but if you're dining with meat-eaters you can try the mashed potato, which is really good too.
While I'm sure you could get something walking in, I would suggest calling first. It only takes a few minutes, and you may have more options.