The vast majority of the hotels are located on Santa Cruz and it is possible to do day trips by yacht to some of the small uninhabited islands close to Santa Cruz (about 30 minutes to 2 hours away). Since the park service limits the number of people that can visit each site each day, they determine which island can can visited on a given day. These Galapagos tours can start on any day and can be of any length.
Spending nights on several different inhabited islands (island-hopping) and doing tours on those islands is another option. That could be good for families or people with disabilities and for those who want to mingle with the locals, but access to the national park sites is kind of limited. It is good for multisport tours.
Since probably one of the main reasons that people do a Galapagos tour or cruise is to see the wildlife, you must consider that not every species can be seen on every island.
Of course there are all sorts of cruise options. Itineraries can be from 4 to maybe 15 days and boats can range from small yachts to ships with a 100 person capacity. Levels can be from budget to deluxe. The smaller boats of up to 16 guests normally have 1 guide, so everyone does the excursions together. The larger ships might have up to 7 guides and they will divide the guests into groups. It is possible that they might group people with specific interests together - such as those who want to be more active or those with kids. Some even have glass bottom boats, which might interest those who don't snorkel, but want to view the underwater life.
The standard cruise options used to be 4, 5 or 8 days, but a few years ago the park service changed the rules in order to prevent too many people from visiting each site daily. Now you can see cruises of 4,5,6,8,10 and more days. Some weeks a ship might go to the western islands and other weeks they might go to the eastern islands. So if you have particular islands that you want to visit, make sure you choose the right itinerary!