Rinella Fort is a place where you can spend an interesting hour or so. It's the home of the 100 tonne gun which is slowly being renovated. Full marks go to the dedicated volunteers who man the museum and carry out re-enactments of life within the Victorian Fort.
Do try and visit when the re-enactments are taking place, you can even fire a cannon or a musket yourself! Once a year they fire the 100 tonne gun and you can get special tickets to see this. All blanks of course!
Good combined with a visit to the Three Cities.
http://www.wirtartna.org/FortRinella/fortrinella.htm
Oh and Mdina too, excellent views towards the northeast from the walls, quite atmospheric, a playground for the kids and reasonably good restaurants.
Also the Citadel in Victoria on Gozo, small, but some interesting little museums tucked away in the old buildings, you can buy a ticket that allows you entrance to all of them. The prison museum was particularly interesting.
The Upper Barracca gardens in Valletta are a good place to view the Three Cities, lovely views but can get a bit crowded. Small cafe sells snacks and is a good place to visit combined with a trip to the Valletta market.
If you have kids then I recommend you should also visit Splash n' Fun. Quite pricey but a good day out and a great alternative to Malta beaches. Lots to do for small ones, clean, plenty of supervision by staff, lots of sunbeds and a reasonable cafe.
Kids would also like the Playmobil Fun park, near to the airport. Beware the shop! and pass straight through to a small outside play area and inside activities. Plenty of Playmobil to play with and keeps the children occupied for a couple of hours!
If it's raining (yes it does rain in Malta occasionally) there is an amusement arcade inside the Bay Street Shopping complex. Buy tokens and then collect tickets from the games and swop them for prizes! You need about 3,000,000,000,000,000 for anything decent but sweets and small toys are obtainable!
Another indoor attraction - you can pay to use the indoor swimming pools in many of the health clubs that can be found in the major hotels. Can be expensive but definitely an option for a relaxing couple of hours.
Just want to add Slug Pool onto this list. A small rocky bay with clear blue water near to Marsaxlokk. Can get a bit grubby if the wind is in the wrong direction but there is an interesting short walk by the coast.
Ciao
Susan