spending three weeks in Italy in Sept.2024.Have seen most of Italy but never the Adriatic Coast..Is it worth the time and what are the highlights? Thanks for any suggestions. Frank

If you haven't been to Puglia you could easily spend three weeks just there, including Matera (in Basilicata).
Other highlights from North to south below. Some interesting towns and cities are a little way back from the sea, 5/10 minutes up to 45 by car:
Comacchio (and Ferrara and Abbazia di Pomposa)
Ravenna
Historical centre of Rimini, https://slowtravelitalyspain.blogspo...na-rimini.html, San Marino, Verucchio
Urbino
Ancona -Osimo- Parco del Conero - Numana - Sirolo - Loreto - Recanati
Fermo
Ascoli Piceno https://slowtravelitalyspain.blogspo...a-rosa-di.html
Teramo
Vasto
Termoli
Then once you get to Puglia, everywhere is fantastic!

I'd start in Venice and also the mini-Venice of Chioggia
Then the Po national park with some amazing bird wildlife,
Ravenna, Vasto etc
Le Marche, Abruzzo and finally Puglia

during that time you will be able to watch the dishes change with more peasant/vegetarian food coming to the table, wonderfully different cheeses and the wines getting more fruity

Are you cycling because 3 weeks would be a nice time to do that in?