Airport showers.
On a really long trip, if the layover is long enough to do something other than run between planes, a nice, hot, refreshing shower is well worth whatever it costs. A nice shower is so refreshing after sitting for hours in an airplane, crammed in like sardines. It makes the next flight so much more bearable.
While flying to Bhutan recently, I had to go from Chicago to Tokyo to Bangkok, with a two and a half hour layover in Tokyo. I am a very tall person, and after 13 hours on the airplane, with little sleep, I was ready to scream. Or collapse in a puddle of misery. My hope was just to sleep in Tokyo. But after seeing how well the others responded to the showers, I chose to shower also.
The $3 spent on that shower was probably the best $3 I have ever spent. The shower was clean and the soap and shampoo were large enough to be useful. The large shower head massaged my entire body - I felt almost like a new man. I was able to sleep, soundly, for most of the six hour flight to Bangkok.
Airport showers are worthwhile, even if they cost much more... I could see paying $20 or more in the same situation. Or you can sit for hours in an airplane, tired, sweaty and aching.