I did this tour in September 2008 and would thoroughly recommend it. Starts with a gentle walk down to the trams which take you out to Serenity Bay. Then a walk along the coast with some information about the local plant life including the two poisonous species of trees not to go near. I suppose the walk is about a mile and unhurried with a couple of stops to look at the sea.
The Kayaking is a bit more strenuous, there is a quick explanation of things to do and not to do, but you are never in more than 3 feet of water so drowning unlikely. However they do recommend that if you see something in the water you do not both lean over to look, I can confirm the accuracy of this warning.
they are two person Kayaks and unless your really used to Kayaking you need two people to push them along. At some point in the trip you will be against the wind and or tide. Having said that there were people on the tour who had clearly no experience and everybody coped. You can always get out and push. Starts with a kayak up the creek to look at the mangroves then back down again to the beach on the other side of the creek. After that its across fairly open water back to the starting point. We got lucky and got amongst some nurse sharks on the way back which was pretty exciting. I am told these sharks are harmless but didn't try any practical test of that theory. Saw plenty of fish, couldn't tell you what they were though but some were pretty big and a few rays. 1 Baracuda also. Bird life was pretty sparse as it seems to be throughout the island, best place to see the birds is the lunch barbeque if thats what you want to see although there was a frigate bird. Sadly DW saw that and not me.
After the Kayaks its the same walk again following the cycling trail back to Serenity Bay. The tour is not over strenuous but lunch goes down well at the end of it. I had enough energy left to jet ski in the afternoon but the afternoon beach idea is not a bad one either.
Hope that helps