Keep purely “fun” activities to a minimum. If you’re already doing a bridge climb in Sydney, then skip the whale-watching cruise (unless you’re in Marine Studies!). If you take the group kayaking, then skip the canopy walk through the rain forest (unless you’re running an Ecology program). Study abroad programs are not “educational tours” and should not involve students spending most of their time on a bus or plane together as a group. Most excursions should be related to your courses; very few should be purely recreational.
What excursions make sense from a cultural standpoint? If in Rome, the Vatican is a must; if in South Africa, a safari is expected. What visits are a "must" for your site?
What excursions make sense from an academic standpoint? Remember that you are not taking students on a vacation tour. One of the reasons for your program is to enable them to engage in learning that they would not likely be exposed to if they went abroad themselves as tourists. So-venture beyond the typical tourist fare.
What excursions give the most bang for their buck? Consider distance, time, and expense. Is this excursion going to be worth the time and money? Is it relevant enough to the program's core component--the courses--that it's worth the investment of limited resources (time and money)? What would you gain if you didn't go? (A lower program fee? More free time? A slower-paced program?) What would you gain if you did go? (A "selling point" for recruitment purposes? A valuable educational and course-related experience?)