Go with a tour
But really, go with a tour... Don't try to make it happen when you get there. You'll miss out on everything that makes the islands so amazing. I'm all about going it alone, but here, no way. The Galápagos hold vast amounts of knowledge, and there is no place like them. It's not about seeing sites and great vistas, learning about power of nature is why this place is so unique and famous. The learn, you need a teacher that can impart a tremendous amount of knowledge. We went went with Classic Journeys and it was well worth it. When I returned I swapped stories with a friend, a sad soul who went without a tour. He saw some stuff and got a picture with a sea lion but that was it. He didn't learn or retain a shred of information, not to mention, missed out on the more difficult places to get to.
Land v Sea: There are four populated islands, Isabela, Santa Cruz, San Cristóbal, and Floreana. If you go on land, you will probably only see these islands. The upside is that you can go into town whenever you want, stay out by the beaches as late as you want, and take a stroll at your connivence. By sea, you can view more Islands, but you don't have that freedom at the end of the day.
Island Guides: Everywhere you go you need a Galápagos park guide to go with you, even if you are part of a tour group. One guide we had spoke great english and was fabulous at explaining the nature surrounding us. Those are two different things, by the way, a teacher, and an English speaker. The other two guides did not speak English, nor did I get the sense from my broken Spanish they were good teachers. If we didn't have Thomas' (our company's guide) extensive breath of information, it would have sucked, and we would have missed out on all the reasons why you should visit this place.
Camera: If you're into pictures, have a camera that can zoom. Animals are the draw here, and if you have an iPhone, it's impossible to take good shots. A good point-and-shoot should suffice, and camera with real zoom will definitely get you some good shots. At this point, I only had an iPhone, and when I returned, I saw that my pictures were severely lacking. I took epic pictures of western landscapes with just a phone, but here, it was useless. I can't speak for a point-and-shoot as I only recently started using one, but that zoom might not be enough either. Just FYI.