Yes, there are kayaking opportunities in Venice. One of several companies offering guided kayaking:
https://www.venicekayak.com/
Naples to Venice is an ambitious travel plan, and none of those 4 cities offer outstanding outdoor adventure opportunities, unless you take a day and travel well away from the city. When we were in Venice for a month we did find a nice outdoor experience on the island of Sant’Erasmo, which is devoted to farming. It was a pleasant day with 8 miles of wandering around, a small beach, and lots of farm fields (mostly artichokes), but nothing I would recommend for a short visit to Venice.
What I would recommend, if I were helping a friend plan this trip for a group such as you describe, is to skip Naples (you have plenty of Roman history to see in Rome and Pompei) and spend 3-4 nights in the Dolomites before finishing your trip in Venice. Take the train to Bolzano (from Florence) and then a local bus to the Val Gardena, probably the village of Ortesei. Lifts go up to peaks and plateaus on both sides of the town. You can rent mountain bikes, take them up on the Mont Seuc lift, and ride on trails on beautiful Alpe di Siusi. This is a huge meadow surrounded by rugged peaks of great beauty. The Alpe and the hillsides offer mils and miles of paths, trails, and car-free roads perfect for all ability levels. Most places offer e-bikes and they are what you will see most people riding. I guarantee it is an unforgettable experience (on the best way). We have been up there either riding or hiking in 4 separate September trips and had 90 percent sunny days.
On another day you ride the Seceda lift to the top, then hike down to the Col Raiser lift and ride back down to the valley. Despite the rugged appearance of the terrain in these photos, it is not a difficult hike as the trail is well built, with traction footing provided where needed.
https://www.mtb-dolomites.com/seiseralm/en/two-atitudes.asp
https://www.val-gardena.com/en/tours/detail/mtb-tour-val-gardena-alpe-di-siusi/
https://theunexploredbackyard.com/seceda-ridgeline/
You can also reverse the hike and hike up from the Col Raiser lift and ride down. Just don’t try what we did by mistake and hike all the way back down to Ortesei from the top—-that was a brutal day.
Lots of other opportunities for your third day in Ortesei, including an adventure park with ziplines and a ropes course which challenged even my very athletic daughter-in-law, age 45. (Our tween grandchildren and their dad did just fine, and I did not attempt it.)
https://www.coldeflam.it/index_en.php
Or you can spend part of a day lounging by a nice hotel pool or enjoying the spa services on offer at a hotel like this:
https://www.hotel-interski.com/en/recreation/30-0.html
Then head to Venice for another historic and beautiful city.