So you can evaluate. Walk or use boats for MGM/Epcot and among the Epcot resorts. Bus to MK also serves Swan/Dolphin, usually now picks up at BW first so you stop at S/D going and drops off at BW last; however, it is probably still not worth driving because you have to go get your car at BW, go to MK lot and then take boat or monorail to MK. Walking through Epcot to monorail is usually the slowest of any trip because walk is about 20 to 25 minutes to get to monorail, and then you have to take it and transfer to MK monorail or boat.
Bus to AK usually serves BW, S/D and YC/BC, and also stops at Blizzard Beach during the day (both going and coming back). Driving is the significantly faster option for that trip.
Likewise for Downtown Disney, which bus usually serves the entire Epcot resorts, and then during day stops at Typhoon Lagoon.
If you want to go anywhere else than the above (on site or off), it is also best to drive.