On a flight to Europe earlier this month, I took along several mini-size noise-reducing headphones, including a new set from Phillips. Model SHE592 is down to $25 at BestBuy in Canada.
These look like so many other in-ear headphones, which already lower airplane noise, because (1) they act like ear plugs; and (2) the sound is very close to the ear drum.
But these new ones from Phillips have a small box near the plug-end that contains a AAA battery, noise-reducing circuitry, and the on-off switch.
I found they worked very well, although I was sitting in a quieter location away from the engines. It was important to jam the headphones well into my ears, to block out the most sound. These headphone don't block 100% of the plane noise, but more than any other I've tried. (I haven't tried the heavily-advertised Bose headphones, but at $300 I'm not going to try.)
The only drawback to these is that small box with the circuitry. It makes it hard to bundle up the headphones compactly. (Over-the-ear headphones are large enough to contain the circuitry inside the ear pads, but then they are overall much larger than these in-ear ones.)