In our session this past weekend when we played through part of The Three Faces of Evil, the second part of the Age of Worms I tried out using a new critical/fumble system. Two sessions, one being combat heavy, and I think the system held up well, and the kids liked it, too.
I had been using this critical generator from the d20PFSRD, but it relies heavily on ability modifications, which aren't supported in 5e. I found myself re-rolling (or clicking) sometimes several times until I got a result that seemed to fit.
I wanted to keep it simple, and since I'm a fan of Advantage/Disadvantage, I based it on that mechanic. Also, since I like dice (honestly, who doesn't), I bought some new dice to help illustrate this in game.
On a critical hit, roll all damage dice a second time and add any modifiers, as per RAW. In addition, you gain Advantage on your next attack roll. To keep track, I give a steel/silver colored die to the player/monster that got the crit and place it on the table under their miniature.
On a fumble, you automatically miss. You gain Disadvantage on your next attack roll, OR you get the choice of something cinematic (drop weapon, fall prone, ect.). The black/yellowish green die would represent the fumble.
Finally, since I was buying dice, I got some gold ones to hand out for Inspiration.