Hello tipsyletti, there are two sides to Sliema one being the sea front and the other is the harbour both are mostly flat, but the roads leading from the harbour to the sea are all up hill. If you stay on the harbour side you can get the bus to most places in the local area and so cut out the hills, also from here you can get some of the boat trips round the island or harbours. There are a few bars along this side of the harbour as well as some good restaurants, if you want a modern hotel I would go for the 4* Hotel Waterfront, you can find it in the Thomson summer sun brochure or you could Google it, or take a look at the hotel page on this site.
Dave