All depends on your bugged and what you like to see. The Galapagos divides different regions: Central islands, northern, southern, eastern, western. You are able different options, there are cruises, day tours or Island hopping.
You can start from 4 days up to 8-days and 15-day cruises that are able to sail to all regions of the Galapagos in one comprehensive trip.
Also, you can do a combination, spend a few days in Hotel, then take a cruise.
It depends on all on you, figure out what you like to do on the Galapagos Islands.
Very important to mention in the Galapagos Islands the are 4 inhabited Islands, dose are Santa Cruz, San Cristobal Island, Isabela Island and Floreana Island.
Follow Link for information on the Inhabited Islands
You can stay in Hotel and do day trips to the near Islands. Example Stay on Hotel on Santa Cruz Island take a day trip to South Plazas Island or to Santa Fe Island, North Seymour Island, Bartolome Island.
Another way to visit the Islands, You to Island Hooping, this of curse is only to be done from inhabited Islands, dose like Santa Cruz Island, San Cristobal Islands and in between Isabela Island & Isabela Island.
Something very important to mention two only places there where the main airport what you will need to in order to take the plane from the Islands or back to mainland (Ecuador). This is Baltra Island near Santa Cruz Island and San Cristobal; this are only two places what will take you back to the mainland.
You can arrive via San Cristobal and leave via Baltra or the other way. The idea of this is that arrive via San Cristobal, make some day trips near to the Island (like Española Island also different spots on San Cristobal Island), for the next future days; Then take a ferry or small plane Baltra then a bus ride to or taxi to Puerto Ayora on Santa Cruz. Also, with the small plane Fly to Puerto Villamil o Isabela Island, it is not a direct flight, you have a stop in Baltra and later to Isabela.
I still will recommend to do cruise 4 , 8 or 15 days, while a cruise you will see more places in a days and go feather away as near to the visitor sites from the inhabited Islands, such like Genovesa Island, western sites of the archipelago, as a mention in depends on you and your bugged.
Possibilities to go scuba diving, two option:
1.- Diving cruise for 8 days, will go feather away like Darwin o Wolf islands one of the best diving sites depending o the season.
2.- Day trip scuba diving from the inhabited Islands. Santa Cruz (Gordon Rocks, North Seymour, Daphne Minor – Daphne Island, The Camaño Islet) San Cristobal (Kicker Rock-León Dormido) Isabela (Tortuga Island). There are many places more on this Inhabited Islands that I did not mention.
Other option to do in Galapagos is called Pesca Vivencial this is can be offered only by Galapagos fishermen licensed for this activity. And found tours at the inhabited Islands.
The Galapagos Islands can find so many things to do from: Biking, Hiking and Trekking, Kayaking, Snorkeling, Diving, even snorkeling with Sea lions and Penguins.
At the Galapagos Islands find so diverse things like: Active volcanoes, Flourish beautiful beaches, stunning wildlife, awesome underwater world, delicious Food …And much more. Why not to try, you won’t regret.