We do a basic plan then divert to suit the weather always worked for us, maybe book first night then find accommodation as we go.
Which doesn’t really work if you leave home with the intention of going to let’s say, Slovenia. Unless you have weeks away, you’ve got to keep heading towards the country, as zooming off towards the Pyrenees (as it looks like the sun is shining there) ain’t going to work. The same applies on the way back.