If you do opt to go for day trips. I would recommend the following.
Snorkelling here is amazing, you will spend an hour within a cove used by huge quantities of sea lions which as long as you keep your hands to yourself are always inquisitive and keen to play. I promise you will not forget these guys!
The landing also has a lot offer; sea lions can be found loafing all over the island tops, both marine and land iguanas are here as well as the Galapagos hawk and the Santa Fe mockingbird.
Snorkel Kicker Rock – from San Cristobel
This is a snorkelling day trip to a submerged islet. Water around the islet varies between 5-15m and offers spectacular snorkelling. You will see huge shoals of bait fish and as a result the area is used by good numbers of black and white tip reef sharks, Galapagos shark and if you are lucky hammerheads also. We were also lucky enough to see double figures of green turtles, spotted eagle rays, golden cownose rays and sea lions.
North Seymour –
If the frigate birds and blue footed boobies are breeding this is a must do. Watching the Frigate Birds displaying is like nothing you will ever see again and the blue footed boobies are also stunning animals.
If you are based from Santa Cruz this trip offers the best opportunities for seeing the Galapagos penguins. You will also visit a mangrove area which is used as a hangout for white tip reef sharks and on your way back you will stop within a cove used by good numbers of green turtles.
Day Trip Around Santa Cruz.
There are many activities available on the main islands of Santa Cruz and these can be done very economically. Many of the tour companies around the islands will try and sell you a package where you visit 4 of these in a day and pay a lot of money. For me, this is a rip off. Whilst walking through the town you will see white 4x4s everywhere, these are taxis. If you decide on which activities you want for the day you can ask a driver to chauffeur you between these venues over the course of the day. This allows you to prioritise which activities you want to do and also saves you massive amounts of money.
I would recommend visiting one of the Giant Tortoise reserves, Las Grietas and the Lava Tunnels. For me, the twin craters are just holes in the ground.