Ask her what sounds good to her and go out and buy it. I liked sour candy, lemonade, etc. But it's different for everyone.
Buy a movie for her to watch while she's not feeling good. Offer to help her get chores done, etc.
Honestly, just saying "I'm really sorry you're not feeling good. That sucks" can be nice. Sometimes people can be insensitive about it (implying that we should be happy we're puking our guts out because we're pregnant) so acknowledging that she feels bad and that you wish she didn't feel like that can mean a lot.