Most of the walks in the main itinerary are nice and easy but be warned - the hike in the Sierra Maestra on Day 9 is 5km and very steep in places so bring your hiking boots.
I agree with the warning in the trip notes that Cuba is not a destination for foodies - quality and especially choice is not great, although the food tastes better than it looks and is markedly improved in Baracoa and in the restaurants of Havana. Despite the lack of snack food as we know it, our guide bought plenty of local delicacies for us to share as we travelled, which were of excellent quality (especially the guava jelly fudge).
The optional tours are not strongly pushed so be prepared to ask about them a day or so in advance and make it very clear if you decide you want to go on one.
Finally, this is not somewhere you can buy anything you forget to pack - many everyday items are in short supply, especially toiletries, so make sure you bring everything you need.