Our focus isn't on fun for the parents, but fun for the kids. And we ask the parents to help make an event fun for THEIR kids by volunteering where ever and how ever they can. At the actual event, everyone is focusing on their part of the puzzle to keep the evening going in the right direction and achieving our goal. We always have a core group of parents who sign up for specific tasks, and there are always a few parents who ask if we need any help. If we have it covered, we tell them to enjoy the time with their kids, if we need an extra hand or someone to fill in for a late volunteer, we gladly accept the offer.
I also agree, it's not the PTO's job to make it fun, because--in my opinion--every activity isn't going to be fun. So what does that mean? A parent will ONLY do things because they'll have a good time, and if they don't, they won't volunteer again? I think we should be trying to make it "rewarding" for the volunteers. Because all the challenges, frustrations, late nights, not enough help, so-not-fun crap you might have to deal with upfront to make it happen for the students is always worth it when the parents thank us for a job well done and, more importantly, the students tell the "PTO Moms/Dads" how much fun they had, days after the event/activity took place.
I believe the "fun factor" is naturally visible because (1) we have a group of parents who truly find it rewarding to make it a great year for our students (2) they all know each other either because their children are in the same grade and/or they help out at every event and (3) heck, our Board is just a fun group of women. We convey through words and deeds our appreciation of our volunteers, when we're on-site we shoot the breeze when we get some breathing time, we make sure everyone has snacks/water, gets to take a break, etc. The fun comes in coming together as a school community.
Personally, at our elementary school, if you just want to have fun without the headaches, you probably don't want to be a PTO volunteer.