Hi there. I'm not going on this trip, but I have been to Rabac in early May just like you. The weather when I went was beautiful. Into the late 80's. Rabac is a lovely little village, and the people are so friendly.
Excursions? May I recommend you do Plitvice. It's a long journey from Rabac but the lakes and waterfalls are so beautiful. Make sure you take good footwear too. Also the caves and the stud farm are first class.
Have a great time!
