I like that idea too. (Yes, Sundays were always exempt.) Once I was on my own, I never gave up anything. My whole childhood my mother made me give up my comic books.
In my teens it was probably some snack (how much you wanna bet it was pretzels? I was addicted to those things).
But the thing that annoyed me the most was the dictum that you should be doing it ONLY for God, and NOT for any worldly benefit you might get! In other words, giving up snacks for the side effect of dieting, etc.
They gave out ashes today at work and even though I intended to go as late as this morning, by the time the moment came in the afternoon I was so absorbed in my work I forgot to go.
But supper tonight was bread, cheese, olives, hummus, and (for me) herring.
We always had a meatless day at home when I was with my parents, usually Wednesday, because there were a lot of meatless things we liked, and Friday nights we were always out doing the weekly errands and having (meatless) supper at a luncheonette. Those were the days when every Friday was supposed to be meatless for Catholics. But my mother laughed that it was no sacrifice for us because we all liked fish and cheese and all that stuff.
But I like the idea of doing something extra. I even remember a priest saying something about that when I was in my teens, but it never seemed to catch on with anyone I knew. I suppose it's never too late to start...