I am assuming you will be depth restricted, (20m?).
Malta is a mix of scenic and wreck. A number of the wrecks where intentionally sunk, some of the others are war casualties, Malta suffered significantly during the second world war, the country and its citizens where awarded the George Cross.
Although there is not much of it left the wreck of the HMS Maori it is shallow (16m) and located 100m from the shore in Valletta, this is a war casualty from February 1942. Its pretty, with a good selection of life, a nice shore dive.
The north of the Island, Cirkewwa, is probably the busiest diving area, its closest to the big tourist hotels (although the whole of the island is a tourist area). This is the area most people are taken to dive, especially if they are newer divers.
The wreck of the Um el Faroud is a nice dive (intentionally sunk after an accident in the dock yards), but probably too deep.
What type of dives are you interested in?
The Mediterranean, is not as prolific with life as the Red Sea or Caribbean. It doesn't have any coral, and has been heavily fished.
On a side note - the Valletta harbour cruise is worthwhile (I recommend the smaller boats/operators).
Medina is a must see, Fontanelle tea rooms is worth a visit (the cakes are excellent), if you visit, and you get a great view of the Island. Its also very pretty at night.