At first glance, the Stari Grad Plain is not particularly interesting - just a large amount of farmland. But, after exploring it for a day, I have a new appreciation of the site. The agricultural practice here are really interesting and seeing them up close with local producers is lovely. But, even more than that, the Stari Grad Plain should be seen as the backdrop for some wonderful exploration.
My biggest tip is to hire a bike and use that to get between to all the various things to do around Stari Grad. The plain itself is criss-crossed by paths that you can ride along. I found it so peaceful to pedal through the farmland, looking at the dry stone-wall boundaries with the olive trees or vineyards in between.
Throughout the plain are small chapels that are worth stopping at. There are also a few producers and restaurants that you can visit (check in advance if they'll be open or if you need an appointment). The coastline at the edges is also stunning.
But it's the towns I really liked. Vrboska is small but charming, Jelsa is great to sit for a meal or a drink, and there are some interesting sights in Stari Grad itself - I would particularly recommend the Stari Grad Museum, Tvrdalj Castle, and the Dominican Monastery.