I like Kinnie on its own, but it's a good mixer. Diet Kinnie has a similar taste. Available at Amazon if you can't wait.
Anyway, I'm always in Malta in April (I was married in Malta in April 2004, and like to return to the scene of the crime.) The weather can be slightly variable. It was gorgeous this year for the first part of April (high 20s C), then overcast for the second (but still about 20). However, it should be considerably better than the UK.
Local strawberries will be in season. You might want to visit the Mgarr strawberry festival if you want some more populist culture.