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.
With 48 hours, Split is simply too far. How about the Soca Valley? It's not the seaside but it has plenty of outdoor opportunities - loads of hikes in the national park, waterfalls, swimming opportunities, be it in the Soca or the Nadiza River, paragliding, ziplines, kayaking, rafting, canyoning, mountain biking, etc. It also has history - it was one of the major battlefronts in WW1. You could easily sleep at a hostel or an agriturismo, which don't charge a fortune.
After you leave them at marco polo, i always recommend a stop at aquileia just off of a4. Amazing mosaics from the 4th century. Lunch is available on the grounds
The time that I was driving around Slovenia, of an August maybe ten years ago, accommodation was easy to find. When I asked a rural innkeeper, "Where is everybody?", he said, "You'll see when you get to Croatia." And I did, bumper-to-bumper on the coast road on an August weekend. I'd expect heavy traffic in late-July, too, as northern Europeans arrive for beach vacations.
Drive to Ljubljana, take a hotel, and enjoy the city. It's not far from Venice. Do not go to Croatia, stick with Ljubljana, maybe add Lake Bled. If you wish, go to Rijeka coming or going.
I agree with Paul...stick to Slovenia...July in Croatia is incredibly crowded and there is lots of traffic. You also simply don't have time to do both, even with 48 hours. Go to Ljubljana and out to Lake Bled and the surrounding Julian alps. Perfect for two days.
You just go to Slovenia with Ljubljana, Bled, Soca Valley. Plitvice are out of question, not enough time. Buy Vignetta for a car in Slovenia which allow you to drive on motorways and also IDL is recommended.