Hello Welshman 1
im coming from Split and i can maybe help you with informations.
First need to say that you an see many many free things for free. Ofc, some you will need to pay but in period when you are coming is post season so that parts will be or close or not that expensive.
For example, in Split you can see Dioklecijan palace, mostly for free, Peristil, or Sustipan,,,
If you move 25 km further from Split on way of Dubrovnik (bus ticket in one way is 16-22 kn , depends bout bus company) you will find Omis. Omis is really small but great city, surrounded by mountains, river and sea. A lot to see (heritage mostly) and lot to do (hiking, biking, zipline, rafting, canoeing, climbing,,,,)
Some thinks will not work, same as Split cause of "death" season overthere and if you ask for a night life- forget it :)Omiš is not for that!
but again further cca 25km from Omis is a bit bigger city Makarska; known as "night life city" but also a lot to see and do.
Perhaps; nature park Biokovo. 1764m mountain up "to the sky" :)
But you can reach it by car or hiking, biking,,, need to pay 50kn of entrence.
Near entrance you will find botanic garden (in case you like that stuff).
Top of Bikovo gives you AMAZING view on Croatian Islands on one way and on hinterland on other way.
And what i like the most; wild horses. But you need to be lucky to see them. Also, this part is the best in winnter when its under the snow but even now its great. If you decide to go, try to find sunny, cold day. And in case you will hike take a property shoes and lot of water :)
There is really a lot to see and do,,, if you like i can help you with info and directions. Let me know in private message with that :)
P.s. Sorry on my "english" :D
Take care!
Baky