If you have a whole 1~2 days, maybe renting a car is a good idea. You can drive around and see the whole island in a day.
I imagine you are staying at the Hilton or Westin? There is cheap pizza place called "Ronka" nearby, which is pretty good (but stick to pizza or pasta). Restaurants and businesses close for an afternoon break, so it's good to have a place like Ronka that stays open all day and night.
For dinner, maybe try some of the restaurants that serve good seafood, as there are a lot of fishing villages on the island. Otherwise, I don't know if you want to waste your time in a greek restaurant while in Malta, but there is one really good one called "Bouzouki" in St. Julians (walking distance from Hilton, Westin, etc.). The bread and lamb shank is good. Avoid the touristy restaurants like Paparrazi, which are just expensive with mediocre quality.
There is really not much to do, but just soaking up the atmosphere of the historic sites, and crazy people. That's why I recommend driving, so you can say you experienced trading paint with the worst drivers in the world
