Manda - I'm like you in that it's hard to get the balance right between planning (so you get in everything you want to do), and being spontaneous and going with the flow.
For dinner Saturday, if you are going to the Seastar for the Sat party, I'd suggest you have their buffet for dinner - and save the "sunset view" dinner for Sunday. For Sat, you could do Rick's for the sunset and just have drinks, as everyone is suggesting not eating their high-priced food, then just an easy walk from Rick's over to Seastar to play in the pool before dinner, eat, drink, and be Irie (merry). Unless your Jamaican friend doesn't drink, it would be better to leave the drive home to the Seastar shuttle.
Regarding getting $JA - as earlier poster suggested, exchanging them at CCLP would be the simplest. BUT be sure to ask them what rate they use first - hotels are known for not giving good rates but CCLP could be different. Otherwise, if you will be getting into Negril early enough to get to the cambio before they close at 5pm - the closest one I know of to CCLP is the Time Trend cambio on the beach road in front of Bourbon Beach. If your friend is driving and not familiar with Negril, as you drive south from CCLP, just watch for the Time Square Duty Free Center on the left side of road - the cambio is across the road in a 1-story building with iron gates. They can exchange cash or travelers checks and you will need ID - most use their passport. If you will be later than that - you'll just have to find one in MoBay, or another town like Lucea, along the way. Since your friend is Jamaican it will be easy for him to ask and find one. Get enough cash to last until Monday, as all the cambios/banks are closed on Sunday's.
If you just need to hit an ATM - you would go to the town center and easiest ones to find are at the Scotia Bank (across from the Burger King). I'd suggest just withdrawing $JA, instead of $US and then changing them. Also, doing that you don't have to worry about the 5pm closing time of cambio.
