Hi, you can put just about any fruit in there, but I love using mixed berries and blood plums (strawberries, blackberries, raspberries, blueberries, cherries etc - fresh or frozen - whatever you can get, even if you only pop 1 or 2 berries in at a time it all adds up), you end up with an almost black syrup. I only ever use cheap booze, and as well as rum you can add any spirit: vodka, brandy, bourbon, grappa even whiskey etc, you could even tip in any bits of (fruit) liqueur you may have in the cupboard (avoid anything a "herbal").
Taste it as you go.
The best way to improve it is time, the longer you leave it the better it gets.
Another tip is to start another jar ASAP, so you have a backup and it'll be ready for your next event.