Hi Tomasz,
I'm very glad that you have chosen Croatia for your summer holiday, and I have few places to recommend, but let me think about it for a while. The main problem with your wishes are that detail concerning walking distance to beaches, because the closer you are, the price is higher. Please send me a PM with details for what kind of accomodation are you interested in, hotel or apartament, for how many days and in which period, and of course price range which is acceptable.
You won't go wrong with any part of Croatian seaside, and you have historical or natural beauties everywhere you look
Regards, K.
It is not so important how high can someone jump, more important is who is the last one standing