As you are staying in St Julians, i would say Divewise at the Dragonara complex in St Julians would be your best bet. As for eating out cant help you there as i always stay in St Pauls Bay. With an open water ticket you will get a lot out of diving at Cirkewwa where there are a number of dives in your range and access is good. Also Ghar Lapsi and even the reefs at Wied iz zurrieq. A trip over to Gozo is also worthwhile. To be honest there is lot you can do, and you will be hard pushed to do it all in one trip.