I'm currently supposed to be planning my 5th HS reunion. Someone else took the reigns though and since I'm busy with school they can go right ahead.
My plans as they were though was to have something very informal. I graduated with under 150 people in my class so my idea was go to a park, reserve a picnic grove, bring kegs, and have a BBQ. It's informal, cheap, and I believe a decent amount of people would show. Plus there is a play area for all their kids.
Hopefully it won't be too formal, 5 years really isn't a big deal.