At the end of July I’ll have some time to explore on my own after dropping the rest of the family off at Marco Polo in Venice. I’ll have a rental car and will return to Venice to catch my flight 48 hours later. I don’t mind long days and driving and I think I would go as far south as Split, but don’t necessarily need to. Would love to visit Plitvice Lakes National Park, but I think that might be a little too much to fully enjoy it. Will probably stick to the coast and not go too far inland. Interested in outdoor adventures: moderate hikes, waterfalls, swimming holes, unique beaches, snorkeling, cliff jumping, paragliding, ziplines, etc. Also enjoy exploring interesting towns and the architecture and historic sites. Please let me know your recommendations, including modest, clean, secure, lodging with parking. Thanks in advance for your help.
You'll be visiting at the peak of the summer tourist season. Personally, I'd stay away from the coast at that time of year. You might try to see the beautiful seaside town of Piran in Slovenia, but I'd skip a border crossing to Croatia and back with such little time - could be backed up in the summer at the borders.
I'd probably stick to inland Slovenia: Lake Bled, Lake Bohinj, Triglav National Park. There are a few waterfalls you can see. Some that come to mind: the waterfall in Vintgar Gorge (near Bled). The watefall (Slap Kozjak) near Kobarid, a historic town with World War I history (Hemingway spent time there during the war and later wrote about it in "A Farewell to Arms.") There's another waterfall close to Lake Bohinj - didn't get to see that one.
You could spend one night in any town in Slovenia that strikes your interest. Ljubljana is a lovely city with beautiful architecture - more charming than anything else. Won't you need to be somewhat close to Marco Polo for the second night to fly out?
Stick to Slovenia - you really need much longer than 48 hours to go into Croatia.
There are some nice coastal towns in Croatia which are about a 3 hour drive from Marco Polo. Look at Dramalj Beach; we did some fun cliff jumping into beautiful Adriatic waters. Also, check out the town Jadranovo, also in Croatia. Funny, we did not encounter any American tourists but many European travelers. We loved it! Room reservations through Flipkey.
You underestimate your travel times taking into consideration that Croatia will be jammed with tourists at that time and there is a major border crossing. You cannot approach a European road trip like you would an American one. If you want to go to Split, fly there.
I agree that Triglav sounds like a good fit for what you like.