Yes. For those not familiar - while standing (for a start) suck your navel back towards your spine. And hold it there. Keep your buttocks flexed, your pelvis tucked slightly forward.The good ol' Stomach Vacuums. They are absolutley a solid workout to do, and you can do them anywhere. Just also be careful not to pull a muscle or excacerbate a hernia if you have one.
For anyone curious, they help keep the gut and pelvic floor stable, and bodybuilders use them to help keep the waist narrow. An extreme example of it is when a bodybuilder hits a vacuum pose.
It's something to do anytime you're waiting in a line, hanging out, waiting for water to boil on a stove etc. It utilizes time normally wasted into getting some work in when you normally can't. Those times add up over the years.
Once you get used to the exercise - do the same, suck your navel back towards your spine. Only this time breathe properly by bringing air into the belly....and don't let your stomach expand. That will really work the transverse abs, and maybe drive you crazy if you let it.