April/May should be brilliant. The beaches in the North are great and very clean. Apart from activities offered at the hotel, there's not a whole lot to do in the North though unless you are into the night life which Grand Baie is good for. The closest attraction is Pamplemousses botanical garden which is worth spending a few hours in. Port Louis is worth spending a day in for the markets and general wandering round. It is a very hectic place though.
You should definitely venture down south to the Black River Gorges National Park, Chamarel, Le Morne as the region has some spectacular views. In fact, if you don't mind hiking, they've just opened a trail to climb to the top of Le Morne.
Also, enjoy the local food as much as you can. I know some people tend to stay in their hotels for lunch but I'd recommend eating out. It's cheaper and more authentic.