Hi tarry, we used to go to Jersey when the kids were young...but that's over 20 years ago when Bergerac was all the rage. The last time we went was for our 20th anniversary in 2000 and I must admit it had got a bit run down then as regards tourist attractions - but I know there has been lots of development since then round the harbour etc.
Offshore from St Helier is Elizabeth Castle, which you can walk out to at low tide, or they have sort of amphibious "ducks" to take you out there if the tide is high. The castle is quite good to wander round.
You can also walk along the coast from St Helier, or get buses.
http://mybus.je/
There are lots of places to visit depending on your tastes
another castle, Mount Orgueil at Gorey
the Zoo, now called "Durrell" but that's away from St Helier.
There's a great beach at St Brelades, but that might not be brilliant in March!
There's a pottery, German underground Museum (bit creepy) plus other German occupation stuff.
If you get into the countryside I expect the spring flowers etc. will be very pretty.
When we used to go, coach trips were popular. There's still one company operating
http://www.tantivybluecoach.com/Home/
The advantage of a coach trip is that you get a seat to and from your destination (not that the buses will be jam-packed at this time of year), and a commentary so that you learn more about where you're going - but it depends on your budget.
We have never been in March, but you will probably find the weather quite changeable - that's a good thing because if it starts off raining it may well clear up later in the day. Having said that, when we were last there I had a warm coat on all the time while we were out and about....
However the island itself is quite charming - hope you have a good time