Hi Angela! Choosing one park can be difficult so you may want to research the attraction pages for
Magic Kingdom,
Epcot,
Hollywood Studios and
Animal Kingdom. The decision on which park to enjoy is going to be a family preference. Each park has many highlights and if you like thrill rides or adventures, here are some of my favorites:
Magic Kingdom - Space Mountain, Big Thunder Mountain Railroad and Splash Mountain (note: Splash Mountain is closed for enhancements and will reopen on or about March 19th so that may be a factor in your decision).
Epcot - Soarin' and Test Track
Hollywood Studios - Tower of Terror, Rock 'n' Roller Coaster and although not a thrill ride, Toy Story Mania! is a TON of fun.
Animal Kingdom - Expedition Everest and Kilimanjaro Safaris
I typically purchase my tickets on Disney's website or as part of a package. However, I do know several people who save a couple dollars by purchasing their tickets at undercovertourist.com. If you want to save a few more dollars, you may want to sign up for the newsletter at mousesavers.com as there is usually a special link in the newsletter for an additional small discount.
You are welcome to leave the park and return later in the day. If you purchase a base ticket, you will need to return to the same park. If you are able to swing the hopper ticket, you will be able to return to any theme park.
I hope you and your family have an awesome day in the park!