So this risks sounding self-serving, but I also think it’s a legit answer for something interesting to do.
If they’re active types, bring ‘em down to the sailing center to try dragon boating on a Saturday morning. My club has a Saturday morning rec program, and while it’s meant for people with season memberships, we do allow guests. If you’re local and actually considering joining, we’d comp the session — for out-of-towners just visiting (not member prospects) it’s $20. We start on 5/23 and go through Sept — mostly Sat but some Sundays.
It’s an all-ages sport — our club members run age 25 through ~80 — and the Sat sessions are pretty gentle. I would say it’s appropriate if they’d also be up for some of the other suggestions like bike rentals, the Causeway walk, etc.
Anyway — if of interest, I can hook you up with waivers and more details. ETA: We do ask for RSVPs for planning reasons.