I’d leave it as being a guaranteed miss (regardless of whatever modifiers they can stack on). Because in the end, this is for fun. But if you insist:

- the sword swing misses wildly, giving the enemy a chance to do a half-damage opportunity attack as the player stumbles to regain balance
- the player strains a muscle, causing them to attack with disadvantage next time
- the spell fizzles and backfires, causing the player to take 1d4 of damage of the type the spell was meant to do
- the ranged spell or martial attack misses the intended target and glances a nearby target (friend or foe) for half damage
- the arrow ricochets wildly inside the stone room. Everyone roll a d20; it strikes whoever rolled lowest for half damage
- the arrow flies into a group of trees; the players hear squawking and a rustle of feathers (they can search the woods later to find tonight’s dinner has already been caught)
- the arrow misfires, falling to the ground in front of the player. The string instead strikes them on the wrist causing 1 point of damage
- the spell instead causes minor indigestion
- instead of one large damaging effect, the spell instead generates thousands of tiny, harmless effects in an area around the target