This is a common refrain from players new to 5th edition, Basic/Expert, and 1st edition Dungeons & Dragons. I recently wrote a book "On Downtime and Demesnes" mostly as a tool for me to answer this question. The book goes into great depth into the many options but it isn't very portable. I'm Presenting free portable version here, and permanently available for download from DTRPG.
What to spend money on?
The master list of relevant ways to spend money in fantasy games
Adornment: Clothing is expensive. The character gains 1 experience for every 2 gold spent, up to 100 experience. Fashion is a world unto itself in ways to represent character and values. Players can consider double layered cloaks, knife boots, harnesses, or many other things.
Carousing: Originally proposed in 1976 in the article Orgies Inc. they "waste" the gold by partying, gaining some experience instead of using it to increase their personal wealth. Carousing also matches the pulp and picaresque fiction that Dungeons & Dragons was inspired by. Spend a random amount of gold (usually 1dX times 100 gp) and gain that many experience. On a failed saving throw, an unfortunate event happens, like a city fire, untoward tattoo, pregnancy or other circumstance.
Sacrifice: Player characters can sacrifice gold or other valuable objects (such as livestock, virgins, willing converts, or children) to various supernatural entities, spirits, dark lords, elemental energies, and "gods" in exchange for magic items, boons, and power.
Rumors: Player characters can track down information by taking time and spending gold. For every day they go out and buy drinks (10-40 gp), they have a chance to hear a local rumor.
Healing: They can spend their time resting at a healer or in a nice room to regain hit points.
Scouting: They can pay for some equipment, specialized tools, and spend a week planning to scout the location of their next adventure, giving them a +1 on saving throws on their next adventure.
Training: They can spend their time learning special tricks, feats, increasing their skills or statistics. This may take many months and cost many thousands of gold and may require a specialized trainer to track down.
Working: This is reverse spending. The character earns gold. They will have to explain how.
Crafting Items: They can spend gold to make things: Alchemical bombs, armor, poison, et. al.
Gain Influence: 1,000 gp per week for minor influence, 5,000 gp per week for major influence, 10,000 gp per week for Grand Influence. This is acquiring titles, responsibilities, and favors, allowing the players to gain certain rights (such as hiring mercenaries, owning land, et. al.) as well as having input on political machinations.
Retinue: Characters can have hirelings (torchbearers, pack-apes, miscreants), henchmen (classed characters who work for a player character), mercenaries (soldiers who will fortify structures and engage in combats outside of dungeons), followers (troops and men who flock to the character. They cost nothing but must be provided for), sidekicks (a gestalt team where experience is split 66/33 between two adventurers controlled by one player, specialists (alchemists, engineers, armorers, gem-cutters, et. al.), pets (animal or non-human companions of player characters), proteges (spending 1 gold per experience to create a 'backup' character that doesn't start at 0 experience), hangers-on (Dandelion to your Witcher), Cronies/Contacts (people with a pre-existing relationship to the character) and sages (who turn money into true answers).
Congregation: They player can spend money to acquire congregants. As long as they tend to their flock, each congregant provides 1 gp/month in labor or magical research costs.
Investments/Mercantile Ventures: The character invests a sum of money for a term. After the time passes, depending on the risk and the amount invested, the player character will receive a return.
Lifestyle upkeep: It costs gold every month to maintain a certain standard of living. This collects a variety of costs, taxes, housing, food, and various other expenditures.
Clan Hordes: This refers to any family, noble, organization, or guild. The more the player contributes, the more benefits the clan horde provides.
Research Library: Tomes and libraries and books are quite rare and valuable for magical research, learning new spells, researching unknown magics, creating magic items, and more. In order to perform these tasks, a library of sufficient value must be present. The following items will all require a research library.
Brewing a potion: 500 gp and 1 week per level.Crafting a magic item: Takes 1 day per 500 gp of the item.Spell Research: 1000 gp and 2 weeks per level.Vat Creatures & Crossbreeds: 2000 gp per hit die, 5000 gp per special abilityConstructs and Vehicles: 500 gp per ton for wooden, 3,000 gp per ton for metal.
And last but not least:
Construction: Characters can build castles, keeps, druid groves, churches, mobile cities, giant mechs, underwater or volcano bases, floating islands, dimensional pockets, or new planets or planes.
You may notice many of these integrate the characters within the world. This is what gives depth to the game, and why focusing on the 'non-fun' parts drives engagement.
If you don't want my daughter to be sad and you like this content, support me on Patreon.