Endless Summer isn't really part of the plot - it's just an open-ended scenario wherein you can finish any classes you didn't manage to graduate from before the story ended, and do any optional missions, races or jobs you missed out on.
The Collector (optional) Scholarship Edition only
- Starts: New Conventry, Blue Skies Estate
Stealing bikes can be a pain but it can be done. Simply use the slingshot to take down the adults and townies but watch out for police. Get away until the red meter goes down. Make sure the bike you steal goes to his garage marked on the map. You can only steal bikes that are marked on your map.
Letterbox Armageddon (optional) Scholarship Edition only
- Starts: New Conventry, Blue Skies Estate
This is a simple task of smashing letterboxes. You could also do it without the bike but you need to be quick with the timer counting down. Also follow the path and letterboxes that are marked to make it easier to finish the mission.