Hello Rosie, If you do go to Mellieha you do have a great beach just in case you do want to send some time sunbathing. Mallieha is at the top end of the island so you will not be near the main site of Valletta, to go here I would go on the bus as it is hard to park there, if this is your first visit to Malta you can spend two to three days here (Valletta) as there is so much to see.Valletta was built over 350 years ago as a fortified town and apart from a lot of WW11 damage it is as it was 1572. Highly painted church's/ one of the best Cathedral's out side Rome/The Grand Masters palace/Fort st Almo and its War Museum ( look out for show they put on at the fort on Sundays) Manoel Theatre/Admiralty House Fine arts Museum ( with lots of painting of how Valletta used to look way back when) A walk round the walls and the veiws of the Harbours, along with all the pay to veiw places, go into the tourist office on the left of City Gate as you walk to Valletta.
Or you could go to M-dina another old town in the middle of the island full of history and sites. Take the car over to Gozo and spend the day over there so much to see in Victoria, a Citadel with its cathedral with no dome, but a trompe l'oeil in stead, Museums/ Gozo 360 show/ the Basilica of St George with altar a copy of the one in St Pauls in Rome.
There are so many sites to see in Malta I could fill the whole page, the best thing to do is get a good Guide book and see for yourself.
This should give you a start you will be lucky to see most of it in a week you will be planing another trip as soon as you get home.
Dave.

Thanks for all the information.