This is a Sint Maarten-style spin on an old classic. We bought some Guavaberry liqueur in Sint Maarten a few years ago and haven't found too many uses, but this one turned out great.
1.5oz Bombay Sapphire gin
0.75oz Guavaberry liqueur
0.75oz freshly squeezed lemon juice
0.75oz club soda
Combine gin, Guavaberry liqueur and lemon juice in a cocktail shaker three-quarters filled with ice. Shake for 15 seconds and strain into a rocks glass filled with fresh ice. Top with club soda and garnish with orange slice.