You will likely begin your Galapagos adventure, as I did, by flying into the island of Santa Cruz. Before heading into the town of Puerto Ayora, you will already begin to see wildlife. A finch was abroad the bus which transfers tourists to a boat and I saw a land iguana near the boat ramp.
Puerto Ayora is a large town by Galapagos standards, but it's still a fairly small town and most things are easily reached by foot. Taxis are available for further trips such as the ranches with tortoises. I saw plenty of wildlife in the town itself. Look for crabs, marine iguanas, finches, and pelicans right along the waters in town.
You will also find many excellent places to eat, a wide range of hotel options, and some surprising entertainment in Puerto Ayora. I highly recommend the fish cleaning area both by day and at night. During the day, you will see sea lions and birds there taking scraps. At night, you can get excellent seafood there for dinner.
You can catch a water taxi from the port to other islands, but also visit the port at night when the lights attract a variety of wildlife such as rays, sharks, birds, and resting sealions.
The walk out to Tortuga Bay is also a nice way to spend a morning. You'll see lots of lava lizards along the trail and a few marine iguanas (and their tracks) on the beach. It's a beautiful beach and that's probably the most important feature. If you're really short on time and seeing nice beaches elsewhere, you won't miss much besides some good scenery.
I The tortoise ranches on Santa Cruz are excellent for seeing birds and tortoises. There are two different ranches that I am aware of, but one is visited by many more tourists than the other. Neither of them are bad, but I enjoyed there being less people and I saw a slightly wider diversity of birds at the less frequented ranch. If I remember correctly, I believe it is rancho mariposa that I visited. I had to ask several taxi drivers before I found one that understood I wanted to go to this one and not the more well known ranch (I believe this was Primicias). It's a bit out of the way, so you'll want to take a taxi there.
Las Grietas is a popular spot on the island, so you likely won't have it to yourself, but the water is very clear and it is an interesting spot. You may be able to see fish in the water and you'll see birds along the trail out to it.
Los Gemelos is another interesting place to visit. It is a large crater with abundant vegetation. You will likely want to stop here with a Taxi as it is too far to walk from Puerto Ayora.
I would also recommend the snorkeling tour from Academy Bay in Puerto Ayora. While I was a little skeptical, it turns out to actually be a decent site to go snorkeling. Our group was able to snorkel with turtles and sea lions even if the waters were a bit rough that day. I believe this was the only snorkeling tour of my trip where the sea lions interacted with us.