I've spent most of my life cycling in the rural/suburban northeast USA and recently spent a month cycling in Sarasota FL. I would rate Sarasota and Manatee counties as very good for cycling in general, and the flatness is not as boring as I thought it would be. You just ride faster overall on the flats than in the hills. You don't get the scenic views and the nice descents that you get in the mountains, but no place is perfect. For interesting cycling I think visual variety, smooth road surfaces, and lack of traffic are more important than flatness per se.