If you are staying at Puerto Ayora and thinking of what to do with half a day, NY advice is to book a highland tour. For the cost of USD35 (the cab driver will be the guide), you get to view large sink holes (from collapsed magma chambers), misted forest covered with moss and lichens, and of course, the Tortoise Reserve.
For a fee of USD3, you get to wear a pair of rubber boots. The grounds can get very muddy and the boots are definitely a must have.
I was able to view 7 - 8 tortoises in the 'wild' within an hour or so and get up close to these creatures. The tortoises do not like this peering into their face too much and express their displeasure by hissing and withdrawing their heads.
Do note that the Tortoise Reserve is bounded by farms of all kinds; particularly cow farms - a clear sign that these creatures' existence has to be shared with man.
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