I love the line from a Kenny Chesney song "...I told the bartender, mix rum in whatever you've got..." Funny thing is--it works!
Try 1 part rum, 1 part coconut rum. 1 part OJ the 2 parts of V8 splash tropical punch
Another one I enjoy is 1 part rum, 1 part vodka, 1 part pineapple juice and mango juice.
For a party try just buying an assortment of juices (OJ, pineapple, mango, cranberry...) flavored rums (cruzan has some great ones), dark rum, light rum, cream of coconut and fresh nutmeg to grate and just let everyone experiment.