We are doing something similar to this..Every time a parent volunteers at the school for a PTO sponsored event or PTO meetings or for classrooms (anything really) they fill out a ticket that goes into a large box that we have at the front office. At the end of the year we are drawing 1 or 2 names and one child from the family will get their book rental paid for the following school year. Including those that move on to a new elementary school or move onto the next school due to grade level.
I read this idea (or at least a similar one) in one of the newsletters Tim sent out a couple of months ago, and we've used it twice already as a volunteer recognition thing. We do it a little differently, though. For things where people have to RSVP, like our Bingo night and Family Movie Night, we randomly picked names out of those who said they were coming, then compared them to the volunteer log. If someone in the family had volunteered at any point during the school year, they got special seating. (We roped it off and made a fancy sign that said something like "reserved for our special volunteers, the parents of so and so in Mrs. Smith's second grade class.") The people who have won have just loved it! I didn't think about parking, though--I should add that.
For other school events, like spring concerts, where there is no RSVP, we're just picking a couple of volunteers at random, then calling to see if they are coming. If they are, we'll give them special seating.
I don't know that it will necessarily incentivize anyone to volunteer, but it's a nice, free "thanks!"
I have a new idea to share that doesn't cost anything, doesn't raise any money, but is just a nice thing to do for your parents. For our grade-level concerts this year, we are going to raffle off reserved seating and parking. The winner can stroll in at the last minute rather than getting to school an hour early because our parking lot is jammed and Dad wants to videotape.
1-2 weeks before a concert, we'll send home a flyer (for that grade level only) announcing the raffle. Anyone who has helped at the school in any way can send in an entry. Parents decide if they qualify. The entry ticket (cut off of the flyer) will ask how many seats the family wants reserved. We will select 1 or 2 winners. The night of the concert, we will rope off a parking space for each winning family and the proper number of seats in the front row.
We tried a similar raffle a couple of years ago, with paid entries. The little bit of money raised was a pittance, so we dropped the project. Now we've refocused the raffle on it being a "nice thing to do", rather than a "fundraiser". I'll report results back in the spring after the concert season.