If you're prone to anxiety or tension (e.g. headaches or grinding your teeth) then doing breathing exercises while you're on the go is a good idea.
The way to do this is to breathe in through the nose whilst counting (in your head) to four and out through the mouth whilst counting to five. Try making the space between each number longer with each repetition and if you can, tense your feet muscles as you breathe in and let them relax when you breathe out. Do this with each muscle working up your body (maybe not your face if people can see you!) and once you get to the top you should feel ten times more relaxed.
This is an exercise mainly for the ladies, but also for the men, too. Simply identify the muscle that you use to stop the flow of urine when you pee and tense it. Women are advised to imagine they have an egg in their vagina and they're trying to pull it up and towards their back.
Basically, try holding the muscle tight for as long as you can. It's very tempting to make facial expressions and some people might figure you out, but in fact kegel exercises help to strengthen orgasms, improve the quality of sex and most importantly help women to recover from the common problem of incontinence after childbirth.
This is probably one we've all done, but it's easy to forget how brilliant it can be. Some people even invent games out of clenching the muscles in each bum cheek, such as holding the right one until a car in the fast lane overtakes and clenching the left when you overtake a car in the slow lane, or clenching them in time to music.
Of course, it might be a little bit visible, but if it is then whoever notices is staring at your behind because it looks so fabulous in the first place. If they notice, they'll know exactly how you keep it so shapely and might start doing it themselves!
Okay, people might notice you doing eye exercises, let's just say it's not a good one for when you're having a conversation. But if you're facing the coffee machine or you've got your back to everyone then it's very worthwhile.
Simply focus on one point in front of you and then look as far left as you can for 7 seconds. Look right for the same amount of time, then up and then down, and then round clockwise, and finally anticlockwise. You've just given your eyes a workout! At the start you might feel a little dizzy, and you might want to try these one at a time and close your eyes a bit in between, but as you carry on exercising your eye muscles you could improve your eyesight and will certainly prevent it from getting worse as you age. If you work with computers you should be doing these exercises regularly.
Balancing is not something you have to do with one leg up and arms out beside you. In fact, you can find your center of balance with both feet on the floor. Try practising shifting your weight between both feet and then trying to center it perfectly. This is a great exercise to try when you're waiting in a queue, or perhaps more of a challenge when standing up in a wobbly train vestibule, waiting for your train to reach the station.
Practising your sense of balance will help immensely when it comes to actually doing exercises like yoga and if you know about posture then you'll know how to stand to get the best balance and straighten your spine.