I've been to and think the crocodile farm is worth visiting. The fare should be about the same as to the airport which might be a little expensive. The zoo Tsimbazaza with gardens and museum is also worth a visit and is nearer. The zoo is supposed to open at 9.00 but didn't open until 9.30 when I visited also the night house and the museum were both shut. The snakes are fed live, day old chicks, which some people find upsetting. Some of the animals appear disturbed pacing their cages. Finally there are guides which you might or might not wish to employ but if you do then I suggest you agree the price before you start.
You can get a free street map and advice from ORTANA the tourist office where someone speaks English. The office is at the base of the steps leading to the Place de Independence.
A good walk is to start at the Rova, go there by taxi, and walk down the hill. You can see the church where people were thrown off the cliff and there are good views. There is not much to see inside the Rova - I am told by people who went inside. I didn't.
There is a Museum of Art and Archeology 17 Rue Dr Villette in Isoraka if you can find it and if it is open, which is supposed to be worth visiting. There are markets of which the Mahamasina has everything and a flower market near Lake Anosy.
Also you can walk along the Avenue de Independence which is interesting.
It is good to keep aware of people around you as there are pickpockets some of whom are children. Bradt guide advises the the Avenue is unsafe but it seems safe ot me. The Mahamasina is possible a little unsafe.
Enjoy...