My favorite summertime paddling workouts are upwind downwinds. We get such good wind in the San Francisco Bay Area that it's easy to forget the morning flatwater and save the workouts for the late afternoon when it's blowing... I paddle straight into the wind for a set amount of time or distance and then surf my way back to the start for another lap. It's a fantastic way to get a quick workout and develop technique, efficiency & power in lumpy conditions. Not to mention, it's really fun whether I'm in a sea kayak or on a standup board.
Paddling upwind builds strength and lets me work on timing & stroke mechanics. It's also a great way to reinforce wind dynamics on watercraft and practice the best way to turn into & with the wind. Running downwind requires timing, sprinting, surfing, smiling, laughing, water reading skills and stroke technique among other things. It draws on most aspects of board/boat control as well as balance, bracing and stamina. It's good to mix up the training sessions and paddling upwind downwind creates a very natural & varied session for me. Tonight I was playing around with a new camera & mount, so I got a few shots in as well. And, I couldn't resist raising the sail for a few minutes..