Good strategy to focus. From Yellowstone you can get to the Tetons and back in the same day, though you may not wish to rush it. They are truly beautiful.
Depending on what you want to see, I would suggest a focus on the west/southwest side of Yellowstone because there are more thermal features per distance traveled. The central part of the park has a big figure-8 set of roads, with roads in & out of the park connecting to it.
If you plan to hike the trails (great way to see the park,) we suggest taking along some bear spray. You should be able to buy it in any of the shops in the area that cater to visitors, especially hikers. It looks like a small canister with a spray nozzle, sort of like a fire extinguisher. Then just study how to use it and keep it on your hip, vs. inside a day pack. The chances of your needing it are slim, but it's a good precaution. We like the solitude and silence of the wilds, but if there is a chance of a bear encounter, it's best to make some noise as you hike, such as talking loudly, singing, clapping your hands (vs. the petite "bear bells" that look like big jingle bells & don't transmit sound so well.)
The staff at the park visitor centers are really helpful for advice on good hikes, depending upon your level of ability and how much time you want to spend on the trail.
Clickable map at
http://www.nps.gov/yell/index.htm
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