RAW, Quite a Few, mostly from Feats
There are several Feats that give reactions.
- Defensive Duelist (Prerequisite: Dexterity 13 or higher) When you are wielding a finesse weapon with which you are proficient and
another creature hits you with a melee attack, you can use your
reaction to add your proficiency bonus to your AC for that attack, potentially causing the attack to miss you. (PHB, p. 165)
- Mage Slayer: When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against
that creature. (PHB, p. 168)
- Sentinel: When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn’t have this
feat), you can use your reaction to make a melee weapon attack
against the attacking creature. (PHB, p. 170)
- Shield Master: If you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you can use
your reaction to take no damage if you succeed on the saving
throw, interposing your shield between yourself and the source of the
You could also use the Magic Initiate feat to gain the Shield spell, which takes a reaction to cast.
There are also conditional actions you can take depending on what your character is doing, for example, if your character is mounted during combat:
If your mount is knocked prone, you can use your reaction to dismount
it as it falls and land on your feet. Otherwise, you are dismounted
and fall prone in a space within 5 feet it.(PHB, p. 198)
XGtE also gives (optional) rules on identifying spells that declare that identifying a spell as it is being cast requires a reaction. Although this requires an Intelligence Arcana Check (DC 15+level of spell), it is theoretically attemptable by any character (though if you are the same class as the caster, you have advantage on the check). (XGtE, p. 85)
In my experience though, the vast majority of reactions will be spent on what you already mentioned: Readied Actions, and Opportunity Attacks.