I went for a couple of days back in '04 (also in February), it was awesome! Can't vouch for any places to stay, though, since we were visiting family at the embassy and got to stay at the old ambassador's residence (from when the island was its own country). Good idea to skip Dar entirely, not much to recommend. I stayed at the Serena in the Serengeti, and it tried really, really hard, but it was probably the worst place we stayed on the trip (most expensive, too). If you're looking for a hotel near Ngnorongoro Crater,
Gibb's Farm is
the place.
I'll second all of Bagor's recommendations for
Zanzibar. I really liked the Prison Island, but I love tortoises. You can get some snorkling in on that trip, as well. There are lots of different spice tours, very interesting if you're into food, or plants. You might want to check the restrictions from Agriculture before you go - I brought back a ton of spices that I know were illegal to import - I declared them and they didn't take them away, but I bet if the ag official had been awake they would have.
My favorite trip on
Zanzibar was to the Jozani forest about half-way down the west side of the island. It's probably the closest to a nature park on the island, old-growth mahogany and a really neat boardwalk through the mangroves. Also one of the last refuges of the endangered red colobus monkeys. There's a visitor's center that will hook you up with a guide. Here are some pictures I took at Jozani:
Have a great trip!