the harder the armor you wear, the more uncomfortable it gets over time. if you wear armor used for fights like iron and diamond etc it starts getting itchy and stuff. so maybe you could wear leather armor to get more sanity. or maybe even dye it so yknow its colorful and better for you eyes and sanity, not just all grey and brown. it also gives the leather armor a use.

you could also fly with an elytra to gain sanity, because flying is fun.. not for too long though

making redstone contraptions, placing blocks, clicking buttons and pulling levers all help you stay sane

getting the darkness effect or simply going into the deep dark biome affects your sanity

finding rare ores and loot, doing the archeology stuff with the brush, also make you happy

riding camels, horses etc.