See, that's the catch. The rental IS the easy, not so expensive way, particularly if you time it so that you keep the car over Saturday night.
Taking a bus or car service to those places is going to cost a lot, restrict when you can go and how long you can stay, and requires a lot more arrangement to set up than renting a car on-property would. Of course, if you cannot drive for whatever reason, bus or car service will be the only option, but IMO it is worth it to rent a car for this if you are able to do so.
From the southeast corner of WDW property at Pop, both KSC and Universal are pretty straight shots north. USIOA is on I-4 and KSC is a straight shot via I-4 and the Beachline to Cape Canaveral. Gatorland is straight east on Osceola from the resort, then north on Orange Blossom Trail.