I contribute every year to the Swimming Saves Lives Foundation (this is a great program and I encourage everyone to do it when you renew your membership).
Helped out at swimming saves lives last year and over the years I've helped I don't even know how many recreational and struggling triathletes out with pointers on how to swim like a swimmer. Contribute to the Hall of Fame and Swimming Saves Lives when I renew. Coached a city swim team for 6 or so years. Helped out at info tables at meets & served on the board of my team for two years. Volunteer as a swim buddy/swim angel at local triathlons.
Other than that...nothing
If your club hosts a local meet, volunteer to be in charge of some part. If your club runs a postal, be sure to volunteer to time/count for another heat. I am a meet director, swimmer and coach. For many years i have put on a meet with help on race day but I had never assigned duties. This year I was tired. My workout group stepped up and volunteered for various jobs: arranging for officials, getting volunteers for check-in, etc., getting food for officials and workers, arranging the social, coordinating relays, giving our awards. What a difference it made for me. Each of them definitely "paid it forward" and enabled us to have a great local meet which had benefits for many others.