Once a year you can do an earlier 5K on Castaway Cay. You just need to do one of the races during the WDW marathon weekend in January, and take the 4 night Dream cruise that immediately follows that weekend. You get off the ship while still dark, and do a slightly longer course (the regular 5K done for ever cruise is a little less than 5k). The is the runDisney Castaway Cay 5K Challenge race. This one is not free. Registration for this race next January is $80, but you do get a free race shirt, a snack box at the end, and a metal 5K medal. If you are ambitious, you can do the challenge 5k early, then also do the regular free cruise 5k.