I am going to Gargano in July and would like some ideas.
I suggest reposting in the regular Italy forum where more people will see your request.
There are very few guidebooks about the region. One is Lonely Planet Southern Italy. We went on the RS Southern Italy tour last May and spent two nights in Vieste. Highlight of the visit by chance was the Santa Maria di Marino Festival. We also did a boat tour of the bay to view the caves. July will be busy in Gargano (well, maybe not this year) as it is the tourist destination for Italians. I much preferred Alberobello and Matera. You may want to look at trip reports and scrapbooks on this site.
Enjoy your trip
We drove the Gargano peninsula a couple of years ago. The roads are narrow, very windy with lots of blind curves, and Italian drivers aren't exactly faint of heart--they pass like it's nothing. You can't drive and look at the beautiful scenery at the same time. We enjoyed Vieste for a day, but was happy we weren't staying longer than that. There is a nice seawall area for a walk, and a small city center where we had a quick (and not very tasty) lunch. The town of Manfredonia had potential, with some interesting markets, a pretty cathedral, and a great ice cream place, but it took us so long to find our Airbnb, we didn't have a lot of daylight time there.
You can also to think to visit Calabria region. There is a road named ss 106 that connect Puglia region with Calabria. Calabria it's very affordable and it's not touristic.