It seems like every party we go to geared towards making it easier on the parents - Chuck E. Cheese, McDonald's, etc. I think it is mostly so that they are able to invite the entire class. There is absolutely nothing wrong with that, the kids always have a good time. But, what I noticed was that the kids never really played together. The birthday child would open presents, they would eat, and then everyone sort of went their seperate ways. So, when my son turned 6 we had a "spy" party. He invited about 6 of his best friends. We started off by playing the usual games but set them up as sort of a training camp for secret agents. Pin the tail on the donkey was set up as a bullseye game, a pinata was for target practice, etc. Then I took them inside to get their goodie bags, which had been taken away and hidden. As the adults pretended to wonder where they were I had someone sneak to the front door, ring the doorbell, and leave a briefcase with a tape recorder inside. It had a message sending on them on their first mission to find the goodie bags. So they had to follow clues on a scavanger hunt. They had a great time just doing that but then I recruited my BIL to play the part of the "bad guy" who had stolen the goodies to begin with and gave each kid 5 water "bombs" to attack him with. We had spy themed prizes for each game. Then we grilled hamburgers, hot dogs, etc. for dinner. The kids really had a good time. In fact, it has been two years and those 6 kids STILL talk about it. Mostly, I think, just because it was different from the usual park, Chuck E. Cheese, McDonalds type party and they were able to interact with each other since it was a smaller party with planned activites.
For his 7th birthday party we let him have a slumber party. Again, with only his best friends. We played a lot of traditional games balloon relay races, egg toss and such. We gave them all glow sticks and flashlights and let them chase around in the back yard. We ordered pizza for dinner and I let them stay up late but rented movies for when it was time to settle down. And this party ended up being extra fun because the kids all got to play with the present that they gave (you know how excited they are when their present gets opened and then they never really even get to see it out of the package). And that also helped fill the time. Three of the mothers whose children came have told me that their kids have asked to have the same party when it is their birthday so it must have been fun for them.
So, all of that to say that I have found the best parties to be the more traditional ones with planned, interactive games. I understand wanting to include everyone but it seems to me that the size and expense of elementary parties has gotten out of hand - to the point that they aren't as much fun as the ones we used to have when we were little.
As far as parents who can't leave their child, I've never minded them sticking around. I think they know that they aren't there to be entertained and they usually pitch in to help with the party. I always have a few extra goodie bags or balloons for younger siblings who may show up with the moms who stay for the whole party.
The best advise I have is to talk to your child and find out what kind of party he/she has in mind. Usually, it starts out pretty extravagant but you get an idea of what will make them happy and can set limitations and give them choices from there. The thing to remember is that the more original the party is the more fun it is likely to be - for your child and his/her guests. Plus, it makes your child feel really good when the other kids make a fuss over how much fun their party was.