The island is not so large. It's only around 16 miles from end to end. The island has an excellent and inexpensive bus system but in August, because of how busy it will be there, the competition for seats on any bus will be fierce with lots of shoving and elbows flying. Getting around by taxi is convenient but expensive. Renting a car or ATV makes the best sense, although finding parking for a car in August will be a challenge.
Fira is the island's social/commercial center and is located on the cliff overlooking the caldera. With all the cruise ships and charter flights coming in August Fira will be a zoo. If you want a caldera view I recommend Imerovigli or Firostefani so you avoid the chaos. Imerovigli will be quiet but has the best overall caldera/sunset views. Oia is the most beautiful and romantic village. It will be very busy in August.
This map will help: http://www.motorinn.gr/sites/default/files/pictures/santorini-map.jpg (if needed, click on the map to enlarge it.)
The two main beach areas are Kamari and Perissa. Accomodations are less-expensive there overall. They are popular destinations with the charter flight/holiday companies so they will be very busy in August.
Accomodations range from €25/night to over €1600/night, so without knowing which village you choose and your budget making recommendatons at this time would be a waste of time. You have the option of using the filter system at www.booking.com to search for what's affordable to you.
As for activities, you can go to tripadvisor.com, find the Santorini forum and click on "Things to Do" at the top of the page in the green strip.