Well if you're going to have a sour, then a Rum Swizzle is a pretty good one.
Dale DeGroff has got 2 recipes, 10 pages apart in "The Craft of the Cocktail".
- 60ml Rum
- 25ml lime juice
- 25ml simple syrup
- 10ml Velvet Falernum
3 dashes Angostura
vs
- 50ml Rum
- 15ml Lime juice
- 15ml simple syrup
- 8ml Velvet Falernum
- One dash Angostura
assembled in a highball glass filled with crushed ice and swizzled.

Almost the same but subtly different proportions.