With one day, you really don't have time to get much beyond Tana.

In terms of sightseeing, the lemur park is a good choice. If you want more of a cultural experience, you could go to Ambohimanga, which was the fortress of the 18th-19th century Merina kings. (It is now a UNESCO World Heritage Site).

As for hotels, the traditional recommendation is that the Hotel Colbert is the best in Tana. I really liked the (much cheaper) Hotel Sakamanga, which also has a particularly good restaurant. A bit out of the center, you might consider the Irianja Guest House, which has extensive gardens (with a focus on traditional plants) and serves an excellent multi-course dinner of traditional Malagasy foods.