My feeling is anything you can do on a shore excursion with the ship, you can do cheaper and more efficiently by yourself. You can bypass the crowds and get right off the boat when it docks (while the excursions are still gathering upstairs). Taxis will be waiting if you need to get to a sight.
Your mom is 70. If she has difficulty getting around, the ship's excursion may be good; they'll wait for you. If she gets around well, however, consider how well you can do on your own - rather than sticking with the excursion that's waiting for the people who don't.
I always check out what's on offer through the ship. Then I look at tripadvisor to see what the top sights in town are. After that I figure out which one(s) I want to see and how to do it myself (walk, taxi, bus, tram, train, etc.) making sure I give myself time to get back to the ship.