Do you have any advice for potential travellers?
Take an umbrella! It will rain, you can't avoid it, but waterproof jacket did not help me much, you can at least take pictures under your umbrella.
The tap water is perfectly drinkable, but if you do find yourself feeling unwell, it's a good idea to switch to bottles for a bit.
Make sure you pack plenty of changes or clothes, or be prepared to launder them on the way. The humidity makes it hard to dry things.
Take advantage of the luxury optionals on offer, The Ecothermals hot springs at Arenal is wonderful, as is the spa at Savegre.