Remember that only Disney buses can drop you off AT the Magic Kingdom. If you get a cab/Uber/Lyft or drive, you will be let off at the Transportation & Ticket Center, and have to take the ferryboat or monorail to get from there to the Magic Kingdom.
Some people try taking Uber/Lyft to the Contemporary and walk from there. Some people think you need to have a dining reservation or mobile order at the Contemporary in order for the guard at the shack to let the Uber/Lyft enter the Contemporary property. If it's mobile order, it can be cancelled upon arrival. Some people think that mobile order won't let you get past the gate. I don't know what the truth is, having not tried it myself. You can now walk from the Grand Floridian to the Magic Kingdom, so maybe it would work from there, but it's a longer walk than from the Contemporary.