This app was mentioned in 4 comments, with an average of 1.50 upvotes
I had a shifter monk who, by the rules for bonus actions, couldn't make a monk unarmed strike and a shifter claw attack in the same turn. I felt like this sucked, so I ruled that you can only gain bonus actions from one trigger per round but that trigger gives any number of bonus actions. It's never come up for other players so I don't know how balanced it is but the monk is more or less balanced.
I use the iCrit and iFumble apps for players rolling critical hits or fumbles. They get a choice of maximum damage or regular damage and drawing a card. Usually they draw the card. The effects are pretty fun and give different effects for different types of attacks so you're never decapitating someone with a fireball.
The above link works on computers. Never tried opening it on a mobile device.
There is iCrit on Google Play Store. You can get it for 2 bucks.
For physical tables, you could buy Critical Hit Deck and Critical Fumble Deck. No idea about availability.
Play with a crit deck, there's inexpensive app versions of the deck (iCrit) for android and ios. Instead of dealing extra damage from crits, you draw from the crit deck which will usually cause things like stat bleed/damage/drain, etc.
Having your mobs start piling on ability damage over the course of the adventure will force rest, as restoration spam will only get you so far (second level spell slot for 1d4, or 100gp material component for regular restoration and a fourth level slot).
Later game, fortification will mitigate most of your opportunities to pull from the deck.
Another good mechanism to deploy are diseases and curses. Both require either time or special magic to get rid of. You could play using the unchained affliction rules if you wanted to make them a bit more dangerous than the default disease rules.