hello Pete, although I have not been to Malta for a long time these are the forts that I have been to so I should think they are still open.
1, Fort st Elmo, a Star Fort seen of the epic battle of 1565 in the Siege of Malta, it was rebuilt after the battle in the same shape as before, you can go here most Sunday mornings when they have a rienactment ether by a group of Knights of Malta or one by the French. This is also the place were the War Museum is, and in the lower fort is were they filmed the film Midnight Express.
2, Fort St Angelo, was the main base of the Knights of St John from when they landed in Malta 1530 till 1571 when they moved to Valletta,
the British moved in 1800 and renamed it H M S St Angelo but is now back in the hands of the Knights you can get in here Saturday Mornings, but only to look at the out side of the place, it does give a good view of the harbour, sea and Valletta.
3, Fort Ricasoli, this is the largest fort built by the Knights, you can get in here but it is not made easy and is a bit run down and has a lot of WW11 damage.
4, Rinella Battery, a British fort built to house one of the two 100ton guns, the other was the Cambridge Battery in Sliema. They have rienactments here with men dressed as 1800 century British infantry, all good fun.
5, While you are in this area there are also Fort San Salvore part of the Cottonera Lines, the Sta Magherita Lines, Fort Valdela along with the Birgu Land Front and Senglea Land Front including Fort St Michael.
6, Fort Manoel should if all goes to plan be open in 2010 after being rebuilt after again much WW11 damage.
7, There is the Bastions of the Valletta and Floriana to look at as well, and then there is Fort Sliema now a fast food place on the Sliema sea front, this should give you a few things to look at if you need any more let me know, not sure of any of the prices as I put at the start I have not been for a few years.
Dave.