Oh wow, congratulations on your hike!!
How hot is it in Venice right now??
The one day pass for the Vaporetto is a good idea, but I would explore the outer islands of the lagoon: Burano, Torcello, maybe a leisurely lunch at one of the osterias on Sant Erasmo or Le Vignole (or Torcello)? And yes, a swim on the Lido (free beaches at either end of the island).
Campo Santa Margherita is the best place for just relaxing with a spritz, coffee or ice cream (Caffe Rosso, and there is also a very good gelateria there).
Fondamenta della Misericordia has a buzzy row of cafes and bars- a good place to sit on the edge of the canal of an evening with a drink, cicchetti (which I suppose you could call Venetian tapas) or a meal. It's close to the Ghetto for a spot of light sightseeing- and Madonna dell'Orto, as pretty a church as any in Venice, and definitely off the main tourist highway.
Your only problem with being there with tired feet is that it's definitely a walking city, so you may end up adding to your step count whether you want to or not!