What I do:

- 10mn warmup before a session (mandatory at my age) www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/Pre-session-warmup?page=1

- 20/30mn stretching 5 times a week (but never before a session)
There was a topic on this here but I cannot find it anymore...
Here is a useful video:

Basically I do the "Gravity yoga" exercises (stretching muscles via the passive weight of your body), but modified to obey the results of the science mentioned in the above video: stretches duration between 30 to 60s, 10 stretches for a muscle group max per week.
And if possible try to resist actively against the stretch at the end of the range, for gaining muscle stability.

- if I do not SUP for more than 3 weeks, some paddling muscles building:
www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/Paddle-Trainer-out-of-the-water?page=1

- At some time I used a home made balance board, but I don't anymore, as I did not see actual improvements on the water.
www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/Mastering-lateral-balance-on-a-small-SUP-board?page=3#2402327

What helped me more is analyzing what could be the issues and working on the water to correct them, e.g:
www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/Mastering-lateral-balance-on-a-small-SUP-board

I think doing your weight or resistance on a balance board or ball is great for core strength, or even just standing on one foot. I agree though for pure balance training don't expect it to help you that much on a surf sup. I got a smaller board back in the summer and I hardly use it since I don't get many days where it would work the best or it it too bumpy. After using that small board a few days though it definitely feels like it is the ultimate sup balance trainer when I switch back. A smaller sweet spot will punish any mistake quickly.

Hi, I find Yoga has a lot of good balance related poses, it's also a good exercise for us more maturer individuals.
A strong core is apparently essential for good balance so I do some basic Pilates to keep that strong.
To keep SUP fit I take an Adventure King inflatable SUP onto the Yarra River and practice walking up and down it. Also do to sharp turns while standing back on the tail. If the river is flowing well I also paddle upriver into the rapids.
Because I find the inflatable rather tippy this helps with both balance and fitness.

Thanks, everyone.
For this week I have been trying something new, kneeling on an exercise ball and increasing my squats. I will see how it translates to skills on the water. I have thought about doing yoga past the basic level that I do each day. I think it would be very beneficial.

For some of the people who are getting knee pain after doing leg exercises, it could be the tightening of the exterior muscle on the leg. I know it sounds weird but I have had knee pain for years and I thought it was from an ACL injury (85% torn) from when I was in school. But I found this video and after doing the stretching the pain went away instantly. It might help someone out there.
--SEAb8rBfNGDTO4rVz&index=25&ab_channel=IvanB

Also about squats and lunges, I go to training on Sundays where we do lunges. The instructor usually says that his glutes get fatigued but for me and others around it's the quads. My wife goes to a personal trainer who has got her to change her squat stance so she engages her glutes more.
I think when I and many other people do squats or lunges, we are doing them incorrectly. I am going to have to test all this out on myself before I can recommend it to anyone but this is the video that I am starting with


Unfortunately, I live in a country where I don't speak enough of the local language to feel comfortable listening to personal training so most of my knowledge comes from videos and websites like seabreeze. If anyone has got anything to add or dismiss my thinking please comment.