Writing and implementing software that helps save lives (including hands-on training / usage).
Simple upgrades / techniques can really improve crisis response, mission comms, common operating picture, and real-time situational awareness for decisionmaking.
Helping larger organizations that perform these actions work in faster ways can also be painful as hell, but the end result is definitely rewarding.
You could do this for a non-profit to help with rescue efforts. However, Department of Defense activities and other first responder-type organizations are especially rewarding if you're closer to the mission end of things and you can see clear results. Defense Digital Service is doing some of this work I believe.
Outside of that, working in/with government to build software that help citizens and improves governance is a really excellent way to spend one's time and is quite rewarding.