My husband and I just hiked Monterosso to Vernazza a couple of days ago. I am not sure of your fitness or skill level but we were shocked at the effort it took, especially in the summer heat, and we are both fit and in our early 30s. Vernazza to Monterosso is supposed to be a bit easier so you may not be as exhausted as we were - I think we climbed 1000 steps at least!
We also had a beach day in Monterosso and rented chairs - 10€ per chair for the entire day. Kayak rentals were available at/near the same location. It seemed there was plenty of free beach acesss as well. We didn't have a locker since we were staying in the area.
We dined at La Tortuga on a Thursday evening and made reservations a few days in advance which secured us the most enviable view the restaurant offered. To us, the view (and the wine) was the best part of our meal and found the food to be not as good as Il Moretto, where we had the best mussels and lobster fettuccine we've ever had. We are so thrilled to have had the view but were bummed to have not enjoyed the food more.
Best of luck enjoying your time in CT! It sounds like we attempted the same experience you are going for, just over the course of 2.5 days instead of one very busy day.