When I was younger, I used to do 100 crunches before bed every night. Why? Somewhere along the line of my maturing I got into the notion that I should try to be the most fit I could be (a good habit to get back into, really), so I made an effort to eat well and do at least a little bit of exercise each day. Doing crunches was absolutely a pain sometimes - if I'd had a workout earlier that day, or if I was already comfy in bed, the last thing I wanted to do was haul my ass off the mattress and try to brain myself with my knees. But 99% of the time I'd get up and do it. I had no one to answer to but myself, and I told Me that if I wasn't going to take up running (cardio is the bane of my physical existence), the least I could do it flail on the floor for a couple minutes per night.
Expectedly, I did build some strong abdominals. Along the way, I also discovered the lazy man's trick to building up lower tummy muscles without looking like a doofus. I sat up.
Somebody once told me a story about this really fat elementary school teacher. The lady sat pin-straight at her desk and had the funniest looking gut; her stomach was flat, but every other part of her body was flabby. It confused the heck out of the kids in her class, because she was completely nonathletic, but had abs of steel. You see where I'm going with this.
I was reminded of this trick while riding the bus not long ago. I wouldn't suggest it for anyone driving (because you should be focusing on the road, so put the damn cell phone away before you kill someone), but I used to do it all the time as a passenger. It works all the lower muscles of your abdomen, as well as your thighs and back muscles. You won't get short of breath or (probably) sweat. Best of all, instead of looking silly and having to wear gym clothes, you can do this in fancy or everyday clothes and you'll actually look classy.
Doesn't matter where you put your feet or arms, so long as you're not leaning on anything. Now straighten your back - not to the point where you feel like a human two-by-four, but straight enough that you can imagine your vertebrae sitting on top of each other. If you feel like you're craning your shoulders or hips into some bird-like position, cut it out. Just relax.
You know how when a bus slides to a halt the people on it
I won't make any concrete promises - "You'll gain X amount of abs in X number of weeks!" - but this is way less work than Kegal exercises and I know from experience that if you do this every time you're in a moving vehicle, plus mind your posture whenever you sit, you'll gain a pretty excellent gut from it.
Now about those crunches.
1) Relax, but sit up straight. Pretend your spine is threaded through a string attached to the ceiling (eww). Fight vehicle momentum!
2) Kegals are good for you even though I pick on them.