I don't know about all of you, but I have a kid who LOVES to get down and run around. I don't want to just let him run free on the plane. So here's my story with a bit of advice.
I flew back to the States in November with my then 17 month old son and my mom (she came to visit us, then we flew back with her for Thanksgiving..my husband couldn't go because of work). We requested bulkhead seats at the airport, but they said unfortunately they couldn't give us those because there were couples with little babies who were going to be using the bassinets that only work in the bulkhead seats. We ended up in the row behind the bulkhead seats, which are just like all the other rows in the way of leg space (AKA there is none!). This ended up being an excellent thing. The reason for that was we sat down with my son in the seat between us. He had a seat to himself and no car seat or anything. I had a backpack I was using as a diaper bag because I just figured it would be easier carrying a backpack around the airport instead of a diaper bag. I put the backpack on the floor in front of my son's seat and because of that, he never realized he could get down. I didn't have to worry about the constant struggle of him trying to get out of his seat and take off down the aisle. It was brilliant! So I suggest if you have an equally adventurous kid, you could try that, make sure you get a regular seat and put a bag down in front so they don't realize they can escape. On our return flight it was just my son and I and we had seats on the side aisle, I put him in the window seat and myself in the aisle seat and did the same thing with the backpack on the floor. Worked then too!
Also, I purchased a CARES harness after reading about them on here and researching them on other sites. I have to say it was the biggest waste of money. My son kept slipping out of the bottom and in order to get it tight enough to actually be snug against him, he was nearly being choked by the clasp on it. He also could very easily unbuckle the airplane seat belt so he just kept doing that. I tried it for the first bit of the flight and then I ended up stowing it in my bag and I never got it out again. He actually did better with just the airplane seat belt, once he understood that he wasn't to unbuckle it!
Good luck. This was our second flight to America and I hated it both times. The upside is that it's only 8 hours, so you know it won't last forever, but there really was not anything fun about it in my opinion.