Rest those legs
If a lot of walking or standing is required, think about hiring a wheelchair or electric scooter. Most places of interest such as museums, theme parks, gardens offer these for free or at a small cost.
Absorb the atmosphere
Soak in the atmosphere of a busy street or piazza by buying a drink at an outdoor cafe. For the small charge you can enjoy people watching and rest your feet.
Recharge your batteries
Schedule in rest days where you can relax and recharge. Having a packed
schedule will only add to your fatigue. Do activities in the morning and return the hotel in the afternoon for a shower and an afternoon nap. You will leave for dinner feeling refreshed.
Consider taking organised day trips. It can give you access to places that may be hard to get to on your own. You will also benefit from the knowledge of the tour guide.
Library in your pocket
Get an ebook reader and download books onto it before you leave home.
Ebook readers are light and save you carrying around books.