There’s no doubt alumni events can take time, but quick-hitting events (note the pun) like this alumni softball gathering at an actual high school softball game could be a hit. For you, picking the right sport and time will be your first consideration. Offer free entry, food, memories, even a few gloves and softballs for a game of catch, give a shoutout to the alumni over the loudspeaker … plenty of possibilities here.
These types of events can reinforce the community that teammates once had. It can also help the current players see the support and lifelong friendships that can develop by being a part of something special in high school. And of course, engaged alumni are more likely to financially support their school, too.
This event likely works best if you can identify a few key alumni influencers from a specific team to help plan and recruit, picking a sport that is setup for an event (for example, golf and track and field would be a challenge), and one that has a strong base of support. Having a popular coach lead the invitation and recruitment effort could be a big key to success, too.