add on suggestions to best of Eastern Europe trip

Do you mean Eastern Europe in 17 days? If yes then it starts in Prague and ends at Lake Bled in Slovenia. If you add some days before beginning (which would be wise due to jet lag) then I would recommend Cesky Krumlov. You can also add some castles (Konopiste, Karlstejn, Krivoklat). You can read about these suggestions in Rick Steves book: Prague and the Czech Republic. From lake Bled you can go to Austria, Italy, Bavaria. Depends on your interests.

Where does it begin and end?

We will be starting in Prague and ending in Bled.

Liz - We went down the coast of Croatia for a few days after the tour ended in Bled. We only had 4 nights so we paid a hefty (about $200) one way drop off fee for a car. If we had more time, we would have taken the train to Zagreb and would have driven from there as we would have avoided the one way fee between countries. We drove from Bled to Split and spent the night in Split. The following afternoon we drove down the Dalmatian coast to Dubrovnik. We spent 3 nights in Dubrovnik. If we had more time, we would have either gone to Mostar or spent a couple of extra nights in Dubrovnik and done a day trip or two. One of those daytrips would have been Montenegro. We also would have liked to stay in one of the tiny towns on the coast between Split and Dubrovnik. The roads in Croatia are great and it was really easy to drive at our own pace and find our way around. I just wish we had more time but I guess we'll just have to go back. Other tour members took the train to Vienna and spent some time there, including going down the river to Melk. Others rented a car and drove through the Julian Alps and spent more time in Ljubljana. Enjoy your trip!

I agree with all comments made by prior poster (Julie) but have a few other thoughts for your consideration. Depending on where you fly home from, here is another option which might be a bit more relaxing then trying to cover the longer distances in Croatia - ie to Dubrovnik. You could stay in Ljubljana for a few days - great small city with beautiful architecture. Then take the train to Rijeka or Opatija in Croatia. We used Opatija to avoid having to find the car rental place in the larger city of Rijeka. In Croatia, rent a car and tour the Istra Peninsula - coast towns and hill towns are great but I suspect like other parts of southern Europe it will be busy in mid July (same would be true for other parts of Croatia). If you have not been to Venice, you could take a ferry from a number of the Istra Peninsula west coast towns to Venice and then fly home from there (unless your return flight is booked from elsewhere). Picking up your car in one town and dropping it off in another in Croatia is possible without a fee with some companies - we used Sixt and picked up in Split and dropped off in Opatija without a fee.
Happy travels.