1. Make it a competitive game. Let the kids have some fun and a reward for working hard at spring ball and conditioning. Most of the kids are here because they love the game, give them a chance to actually play. It would also be a reward to a lot of the depth guys and WOs for working hard and contributing even though they won't see the field in the actual season much. Give the fans a reason to come out and support the team.
2. Have a "Meet the team" session designed to get the fans kids some contact with the players. Give or sell an autograph book with players pictures and space for autographs. You want to build real loyalty to a program, get the children to feel connected to the program when they are young and you have a good chance to keep them. They normally do autographs anyways but make it a big deal for the kids who come to see the spring game.
3.
@Buffnik 's idea for ticket specials. Also a chance to pick and buy preferred parking passes, etc.
4. Have some events for families of students who will be incoming freshmen the following year. Do campus tours, meet the advising offices, financial aid, housing, etc. Give the kids who have already decided or are in the process of deciding to come to CU a reason to connect to athletics while they become more comfortable with the campus.
5. As part of #4 above invite the various departments at the university to showcase what they are doing to the public and what they have to offer. CU Boulder has some amazing things happening that almost nobody knows about. Get these people some recognition and at the same time demonstrate to the the public the value of a healthy CU to the state and nation.
6. Have an alumni event with past players, give them some recognition and keep them connected. Good time for some of the players from the past to connect with and inspire current players. Show that being a CU Buff means something for life. (Have a brunch for these guys in the morning or some other event, show them that they matter. The return will be way more than the cost.)
7. Do athletic facility tours, meet and greet coaches and athletes from the Olympic sports, generate interest and support for the athletic department as a whole.
As the event becomes more interesting and valuable the attendance will rise and importantly it will draw people who may not be part of the hard core fans base. Generate future goodwill towards the program, give other stakeholders in the university community a reason to believe that athletics is on their side and is a help to them and sell some season tickets in the process.
Once the event becomes popular enough it also becomes a great opportunity to bring in kids on unofficial visits and show them that something special is happening in Boulder that they want to be a part of.