Heading from Florida to Nashville in late June for a wedding and then on to our summer vacation. Our final destination is Yellowstone. My husband, 12 yr old daughter and I are looking forward to it and want to do our research. Are there great things to see along the way that we should definitely stop on our road trip to see or should we hop on a plane & fly straight there? If so, what's the best airport to fly into? Plane, train, car, we're open to all suggestions. We are planning to take a week to a week and a half trip. We would like to stay at the Old Faithful Inn unless many of you think that's a bad idea. Lastly, how many days does it take to see Yellowstone? What should we definitely do? It's a big park so are there other suggestions of where to stay as we make our way through the park? Should we definitely see something outside the park? All suggestions are greatly appreciated.

PS. Hubby wants to see CA too but I don't want to feel rushed and miss the must sees in Yellowstone.

The company that runs all the lodging in Yellowstone is Xanterra. This is the website to help you plan. http://www.yellowstonenationalparklodges.com/
How are you getting to Nashville from Florida? I'm thinking you will want to fly to Salt Lake City and drive a rental car from there. There are some closer airports but they tend to be more expensive to fly to and to rent a car from. Old Faithful is great if you can get your dates. If you can't, try Mammoth in the north end of the park or Jackson if you want to see Grand Teton NP as well.
You can easily spend 8 days in Yellowstone and not see everything.

I would also add a couple of days in the Grand Tetons on the way.
The drive up to Yellowstone from SLC via Jordan canyon (I think that is the name) is one of the most beautiful I've ever seen.
You have more than enough for that period of time.
SLC is worth a tour also, since you'll be landing there hopefully.

If you can't get lodging in the park, make a reservation that you can cancel in West Yellowstone and/or Gardiner and keep calling the park for in park lodging. People make reservations a year in advance and then cancel as time gets close.

I also think you should fly. The drive is long, and large portions are not that scenic. And definitely stay in the park if you can, but reservations are usually very difficult to come by at this point. Still, keep trying as emalloy suggests. The Old Faithful area is central and appealing (it's nice to stroll in the geyser basin before and after the day-trippers), so take anything you can get there. For me, in the park is always better, because it takes more time than you might think to get in and out. Plus, the ranger presentations in the park in the evening are really enjoyable and perfect for a 12 year old.

Definitely include Grand Tetons and Jackson Hole in your trip because you have time. Your daughter would probably love rafting on the Snake River near Jackson...our son did at that age.