You start small--a little party in the hospital, a conference room, enclosed dining room, etc., punch and cookies. We had one once near Halloween, so kids came in costume. The parents LOVE it, the hospital can get local news coverage (coordinate w/your public affairs/info office-don't leave them out of the loop, they get their panties in a real twist). Volunteers can help if you have cuddlers.
Hard part is to track down all the 'grads'. Once you have a contact list, send info to families. If you get a good response, the whole thing can grow, so get help. Look for enthusiastic staff members. Eventually, you can use a community park, have hamburgers and hot dogs, etc. We have had Fire Engines and ambulances, police cars for the kids, even pony rides one time. (that is harder than it sounds!)
It can become an annual event. As I said, parents really like seeing the nurses, doctors, cuddlers, therapists. And it is good for staff to see that these kids have a life ( usually a very good one) after their NICU experience.