While traveling in an elevator instead of just standing there and staring stupidly at the numbers going up or down, try raising your self onto your toes and then back on your feet again. Do this several times. Also try flexing your buttock muscles as well.
In fact there are many muscles in our body that we can twitch and flex without inviting the attention of others. Even if others do notice you, its no big deal provided you are flexing a muscle in a decent part of the body. (Most of the other parts do not have muscles any way.) Others might brand you as a health freak but it is miles better to be known as a heath freak than as a sack of potatoes.