Croatia

According to Rick's Croatia & Slovenia guidebook, tourist traffic in this part of Europe is extremely seasonal. In general, the tourist season runs roughly from mid-May to early October, with its peak in early August. The guidebook says that Mid-October through early May is dead as a doornail. Many small coastal towns close down entirely, and anything that's open keeps very limited hours. The weather can be cool and dreary, and night will draw the shades on your sightseeing before dinnertime.
Based on this information, you may wish to consider postponing your trip by a month. Alternatively, you could plan to visit one of Europe's many other fantastic destinations.

I was in Croatia this past April and we had a beautiful time. We visited the Plitvice Lakes which were wonderfully uncrowded and the leaves were just that perfect shade of green. Then headed down to Split and over to Brac for 5 days. Things were definitely not dead - I didn't encounter any closures and it was warm and it felt like we had the place to ourselves. Don't plan on swimming, however. Also visited Sibenik (my favorite), Trogir (go there) and then headed up to Zadar for 4 more days. Again, it didn't feel off season at all - just lots of locals and beautiful spring. Had two kids with us and we all enjoyed our trip.

Basically our trip was the opposite of what is reported above by someone who, I am guessing, hasn't been to Croatia during April.

Thanks for the response. Booked our flight last night.