First of all, it is impossible to experience all of what St Lucia has to offer in one day and it is also what you would like to do. I can provide you with a list of things to do and various St Lucia attractions to go to but you would have to decide which one you want to go to.
I am only going to make a recommendation. If you are coming to St Lucia by Cruise ship, you can take a boat tour around the island and stop in Soufriere to do a few tours like visit the drive in volcano, visit the cocoa plantation to see how cocoa is made and take a bathe in one of the warm waterfalls.
I would then head back to Castries to either do some shopping for souvenirs, go horse back riding, go on an ATV tour, Zip lining, scuba diving or some other fun activity.Which ever one of those you want to do, I'm sure you will have loads of fun. Those are just a few of the things to do in St Lucia.
