One of my players is a druid who would like to be able to change their creature type so that they become an invalid target against spells that target humanoids. First party WotC content (including UA) is preferred, though items from previous editions are also acceptable that can be supported by gameplay in 5e.
The option for using their wildshape to turn into a Beast is an obvious option, as is using spells like polymorph, but I'm looking for mechanics beyond those.
A spell scroll of Nystul's Mystical Aura in D&D5e is the closest I've found so far by allowing the target's creature type to be changed for divination spells, but the target is still physically its original type and may still be valid against spells targeting humanoids. Changing to a plant creature type would be ideal in this situation to further some roleplaying aims, though some "minor" bookkeeping changes like vulnerability to fire are also okay.
Best Answer
Spells that have you assume something else's statistics are your best bet.
Since you're discarding the Druid's Wild Shape, other options include spells like Polymorph, Animal Shapes, Shape Change, Mass Polymorph, or True Polymorph. The simulacrum spells creates a construct clone of yourself, though it's not exactly you who becomes immune to humanoid-targeting spells. As far as I can tell, there are no items that change your typing.
Hide your creature type. Nystul's Magic Aura can mask what your type is to abilities like Divine Sense, as can the Ring of Mind Shielding.
If you find a way to change your race (if you haven't made the character yet, or you get a Wish), consider the Centaur race (a fey).
To avoid being targeted by spells, you can rely on cover (to avoid being targeted by any spell that requires line-of-effect), hide from line of sight (many spells require seeing the target), Globe of Invulnerability (for spells below 5th level), divination-foiling spells (Mind Blank, Sequester, Nondetection, Mordenkainen's Private Sanctum), or items that give you an edge against spells (like Robe of the Archmagi).
Other (out of the box) solutions include becoming undead: killing yourself to be resurrected as a Zombie, becoming a Lich (if you dip in Wizard), or a Vampire.