I went there back in October. Sandown Zoo was a bit poor to be honest, although it wasn't helped by the weather and to be fair the bloke doing talks on the Big Cats was very enthusiastic and quite good. Enjoyed the walk to the zoo down the coast from Shanklin more though.
Speaking of Shanklin, this is where we stayed. Was able to buy a train ticket all the way through from Salisbury where I live, the only changes being getting on and off the Ferry at Portsmouth/Ryde. In Shanklin we found an absolutely fantastic Medeterrainian restaurant for those on a budget called The Last Resort - we're actually planning to go back and eat there sometime in the next year or so.
Carisbrooke Castle is quite good and you get some nice views. However it is a bit of a climb to get there so young children might find it a trek. We had lunch in some pub called Eight Bells, I've never seen such huge portions in all my life.
Had a bit of a look round Ryde but it was a bit touristy for my liking, probably due to daytrippers from the mainland.
Here's some pics (I don't have a clue about photography, just point and click shots):
View of Sandown bay from Shanklin (there's a path you can walk down on the lefthandside):
Gate to Carisbrooke Castle:
View from ramparts leading back over itself and beyond:
Guesthouse where we stayed: