Venice you have been before so probably a good handle on how you want to spend your time there.
If I were to return visit I would do more exploring of some of the lesser traveled backstreets.
A daytrip to a nearby town like Padua might make sense or even Verona if you have never been since you have been to Venice before.
Have never been to Padua personally but would like to see a couple of sites there someday and Verona we have been and felt it was a perfect day trip town.
Cinque Terre: you are staying ample time to see all 5 towns, hike and really see everything there is to offer.
A good August day trip I think would be to take the boat trip to Portfino (only runs in the summer) ; the downside is this does take most all day but with 4 nights I think you have the time. If you decide against this the ferry boat in the Cinque Terre also goes to Porto Venere which can be visited much quicker than Portofino.
In Vernazza the Vernazza Winexperience is definitely a place for pre dinner drinks or sunset I think you should check out.
They operate out of the Gianni Franzi hotel ; same place you would eat breakfast if staying at the hotel changes to a wine tasting bar in the early evenings.
In Manarola drinks enjoying the view at Nessun Dorma are a must
To get to Capri, I imagine you would take a train to Sorrento which will definitely have a quite a few stops/changes in route. Once reaching the train station in Sorrento take a taxi to the marina/dock and catch the ferry to Capri. Not sure the hours the ferry runs but you will likely need a late ferry if leaving from Vernazza that same day.
On Capri, definitely take the chair lift to the top of Ana Capri. If you like to hike walk down don't take the chair lift back down, some great ruins and views are to be had. Explore the downtown with the gardens, etc...
Take a boat tour around the island to see the cliffs from the shore, these boat trips all likely include a stop at the Blue Grotto.
You will definitely have time to relax, there is not that much to do in Capri for a 3 night stay.
Do some Lemoncello tasting in Capri.
While in Positano ; spend a good amount of time exploring the town, it's beach and just walking around the different back roads which look like private staircases, you will discover shortcuts everywhere and find it very charming.
Make sure to dedicate a full day to seeing Amalfi and Ravello. Especially loved Ravello. It is easy to combine the 2 in a day via the SITA bus from Positano though expect the buses to be very crowded.
If you have time the path of the gods is a legendary hike for views and can be accessed from Positano.
