My friend and I were only planning to spend a few days in Croatia, but we changed our itinerary last minute to spend more time in the country by the Adriatic Sea.
If you have time, I definitely recommend renting a car and driving along the coast. Due to airline strikes, I was forced to drive from Dubrovnik to Split to make it to a conference. I'm glad I did. The drive along the Adriatic coast gives the Pacific Coast highway drive a run for its money. You even briefly cross the border into Bosnia and Herzegovina.
We also explored some of the islands off the coast. We spent 3 days in Hvar and a day in Lokrum. It rained for most of the time while on Hvar which nearly made us missed our ferry back to Dubrovnik!
My first visit to Croatia in the springtime did mean less tourists than the summer months; however, we had a lot more rain. Next time, I'll return to Croatia in early September to avoid crowds and more sun.