Hi hettiab.
I've been on the Istrian Coast tour twice (once when the hotel was in Rabac, and since at the hotel at Lovran, a couple of years ago).
It's a really nice tour, and I would really recommend it. The hotel is fairly recently refurbished, and I would recommend asking (if you do book) for a seaview room. I liked the hotel, the staff are friendly on the whole, and I found the food satisfactory, albeit it a little repetitive at times.
As for the excursions? I didn't go to Plitvice Lakes this last time, but I went many years ago and Julia is so right. It's totally awesome, but with a lot of walking. Sensible shoes are definitely needed!
I am more than happy to answer any more questions you may have. 🙂