One of my most successful collaborations is the annual egg hunt we hold at the library. The groundwork for this was laid before I started my position, and we had the first hunt at the end of my second month as a working librarian--no pressure! For this event, I am so lucky to work with the coordinator of our local Family Corner Resource Center. It's a great Wisconsin organization whose mission goes hand-in-hand with that of the library, and if you have something similar I encourage you to seek them out to start some collaboration.*
The hunt is for kids 6 and under, and is held inside the library. We rope a local highschooler into dressing up as the Easter Bunny, offer simple face-painting (pink bunny nose and whiskers), a take home craft, and of course--storytime.
The event was such a hit that we ended up splitting it into two sessions limited to 40 kids, and have to firmly enforce pre-registration each year. The kids/parents are told how many eggs they are allowed to collect, and we ask that they empty the eggs and "donate" them back for next year's hunt. There's always a kid or two who gets upset at having to give the eggs back, but I believe it's a good lesson for them, and we don't actually require them to give the eggs back.
While there are many egg hunts that happen in my area, the Library Egg Hunt serves a special niche audience since there is no competition from older kids and no concerns about weather. This is a wonderful, fun, successful event that has become very beloved in my community. Maybe it's something that would work for you!
*I also work with FCRC to offer family events that supplement school district's four-year-old kindergarten enrichment program, and we are planning to use Every Child Ready to Read 2 for programming this fall.