When I visited the Plitvice Lakes in Croatia, I stayed in Opatija, which is located right on the coast in the Kvarner Bay.
Apart from Opatija being a lovely destination in its own right, it is located so that you could quite comfortably visit two World Heritage Sites while staying there: the above-mentioned Plitvice Lakes and the Episcopal Complex of the Euphrasian Basilica in the Historic Centre of Poreč. However, I think that this holds true mostly if you have a car at your disposal – I was stuck with public transport and did not manage to visit both sites. To be fair, though, I might have, if I had planned a little better in advance.
There is evidence of Optija being inhabited from before Roman times, but it really took of as a hip seaside resort for the rich and the famous in the 19th century. At the time, the area still fell under the jusrisdiction of the Austro-Hungarian empire and Opatija became a favourite resort on the so-called Austrian Riviera. A large number of hotels and villas was built, many in the beautiful Art Nouveau (or Belle Époque) style. Thankfully, many remain and were or are being restored, making Opatija a rather pretty city. One of the aspects I enjoyed perhaps even more is the promenade called Lungomare, which allows you to stroll along the coastline for miles and miles. It actually connects several towns in the area. I was also quite taken with the situation regarding food: There are sufficient numbers of cafes or restaurants, and even food stalls where you can grab a bite if you don’t feel like dining out. When I visited, there were several events on at the open air theatre, and you could regularly listen to really good musicians in the park for free.
In case you’d like to explore the surrounding area, there are several pretty towns strungs along the coast, you could go hiking inland in Učka Nature Park or go on a day trip to Istria. Alternatively, you could visit the islands of Cres or Krk.
Opatija makes for a lovely detour from visiting World Heritage Sites!