We plan to drive from Plitvice to Mostar taking the motorway. What are good places to stop on the way?
ViaMichelin gives three suggested routes, the quickest being near the coast. That one takes you near Zadar, Sibenik and Split.
I haven't traveled along either of the inland routes.
Do be sure your rental car is cleared for travel in Bosnia-Herzegovina.
I have traveled the inland route Plitvice-Bihac-Livno-Mostar several times, it's the shortest one. There are spectacular views at National park Una near Bihac, very close to Plitvice but in Bosnia and Hercegovina
If you still prefer motorway route, nice spot to rest is Krka waterfalls (near Sibenik). However, there is a portion of driving just to get to motorway from Pitvice and also maybe 40-50 km of driving from the end of motorway to Mostar.
If you have more time, and did not visit that part of Adriatic coast, interesting and by far the most spectacular is route by "old" road (not motorway) which goes along the coast, (especially Makarska riviera part).
Mostar is beautifull but also its surroundings, especially Blagaj