Hi Craig!
Congratulations on your upcoming Walt Disney World vacation!
You'll have a great time "hopping all over the World" with your Park Hoppers, and it's very easy to do!
On the Walt Disney World website you'll find plenty of
Transportation Information. In the link I included you'll see the first 'Featured Listing' Walt Disney World Transportation page link, followed by links below (in the orange mouse ears - numbers 2, 3, 4, 5 & 6) that will give you detailed information by park, including Downtown Disney.
As far as efficiency goes, the best parks to 'share' days with are definitely those which are located near one another, saving you that valuable travel time.
Epcot and Hollywood Studios are very close to one another and accessible by bus, or by Friendship boat launch via the International Gateway. It's quite efficient to visit both parks in the same day without losing much valuable "fun time!"
Magic Kingdom and Epcot also can be quite easily visited in the same day via the monorail system. Simply board the monorail at either park, transfer monorails at the Transportation and Ticket Center, and a few minutes later - you're there! There's nothing quite as magical as a few extra rides on the "highway in the sky!"
Animal Kingdom is the only park which is
only accessible by bus. You'll find it more time consuming to try to get here mid-day or to share a day with this park and another since your only option for transport is by bus. That being said, I find a perfect way to end a day at Animal Kingdom, since it closes earlier than the other parks, is to head to Downtown Disney! Make a great dinner reservation and enjoy the sights, sounds and shopping in this vibrant marketplace!
Wishing you a Magical time at Walt Disney World Craig!
Kirsten