Here in Iowa, sure signs of summer:
bicycle riders. Lots of them.
Every year there's an annual bike ride across Iowa (RAGBRAI: Register's Annual Great Bike Ride Across Iowa). Details here. People can ride for one day or all 7 (Sunday to Saturday).
It starts on the west side of the state where the bike riders dip their front tire in the Missouri River and it ends at the east side of the state, where they dip their back tire in the Mississippi. They have a different route every year, and this year it's close to me. Like, in my backyard.
I volunteered in two spots, my hometown (Vinton), and near where I live (Hiawatha). It is so much fun to talk with the riders -- many of whom are from out of state, and many from other countries. The volunteer committee sets up fun events for the riders, there's live music, lots of food, camping, more food, and lots of beverages, adult and otherwise.
What happens when 10,000 people descend on a town that normally has a population of 5,000? Fun! A bit of chaos! And a really good time. People in town open their homes to the riders, everybody gets into the action, and the entire state follows along on television and on media.
I have thoughts to use RAGBRAI in a book plot sometime. Now if only I can get the energy to bike 492 miles in 7 days ...