@Lyn57 no way your guests will obey your 25 degree rule. Especially Americans ( no offense but nobody uses AC so much and open windows so rarely as US people ). I wonder why they book holidays in Croatia if they like summer on 21C... why not in Norway or Alaska?
I have another problem (except electricity bills) my neighbours are complaining of noise from my AC during the night. But there is nothing I can do, guests just don't care.
You can also provide a ventilator. In spring and autumn ventilator can be the only option and in summer, when temp rise above 28C you can give them AC too. And just a sheet to cover bc many use all blankets available and then put AC on 21 C.
I hate AC and will use it only when temp is above 38 C outside, and then just for a couple of hours on 26 C . I love summer and open windows 🙂