Hi.
I just came back from diving in Nosy Be. It's great, like diving in a tank, there's so much fish around. Turtles, sharks, little mantas, dolphins, whales, you name it.
I went diving with Madaplouf. Very competent, great fun to be with Olivier and Alain (who speaks very good english), the equipment is nearly new and the dives very varied, but always beautiful. They've got great customer service. Check out their website (also in english).
I also went on a diving cruise to the Radama islands, which was even more spectacular in terms of underwater topography, sharks (two leopard sharks - yipeeeh) and huge corals. I loved every moment of it.
Other adventure activities? On Nosy Be you can rent quads and go off road. You may end up in a poddle with crocodiles. Or go horseback riding.
On the mainland, there's whitewater rafting east of Tana (didn't do it, not enough water), and in Andasibe there's Mad Arbre (kind of canopying). in Diego I've seen kite surfers, but otherwise the possibilities are still undevelopped.
hope that helps
Berit