Renting a car one way is more expensive than renting it and returning it to the same location.
Often that is the same with plane tix (ie you don't save much over going roundtrip).
So you are paying more to do 2 different modes of transportation and in the end may not see any cost savings at all.
Therefore, I 2nd the suggestion of flying and planning to spend time at the destination with dd.
FTR: I am not a big road tripper, spending hours in the car, so that may influence my decision to you.