This is good info, thanks! 15 mph limit won't work for me, so I suspect I'll stick to the roadway. Using googlemaps street view, it looks like there's some good side road routes from Bird Key to Siesta Key. For simplicity, it seems like I'll concentrate on a route from Bird Key, to Long Boat and then Anna Maria...

I didn't know that the sand was so good at Anna Maria...I'll definitely go there!