Hi
Went on this trip earlier in the year and had a great time.
Would certainly recommend the fish picnic, cruise and bbq to Cres, it was a lovely relaxing day and there was a veggie and meat alternative to the fish.
Plitvice lakes is a must, unless you have difficulty walking. An early start and a long day, but stunning.
Trip to Labin and also Istrian discovery were both very good.
Depending on what your interests are you may or may not want to do the trip to Lipica stud farm and Postojna caves. Another long day, we saw only a little bit of the horses being trained. The caves were impressive, tour underground lasted for about an hour.
Didn't do the trip to Venice as it clashed with the lakes, but those who went had a good time despite it being another long day.
Don't forget to take a few Euro's with you, as well as some Kuna. Trips can be booked using credit cards as well as in cash.
Have a great holiday.