"Can I buy magic items?"
"Is a healing potion the only magic item in the equipment list?"
"Why don't these magic items have prices?"
"What are we supposed to spend this gold on?"
These people haven't read the rulebook.
Reading the Rulebook
I've seen many suggestions. Buy hirelings. Save for a castle. Good, old school type things. But that still doesn't address what the rules say about spending money. Let's look, shall we?
First the logistics, then the secret reward option for gold.
|Can I be a douche in Dungeons & Dragons too?|
Let's say you're an adventurer. Do you want to live in squalor? Probably not. How about Poor? Nope, I'm in this for the money. How about a modest home? What!? For a dragon-killer?
All right, what's that wealthy lifestyle come to?
120 gold, per character, per month.
Why at these rates, a party of four could live off all the treasure in Phandelver for about two weeks! Collecting the "Horde" at the end of Hoard of the Dragon Queen? Those same four adventurers could live off those spoils for around five years, if they never spent any money on anything else. That's some amazing horde that will set up 4 dudes for about 5 years. Real impressive.
You can halve that value if you want to live comfortable, or, more than double it if you actually want to be wealthy. The general idea is that living will cost you quite a bit, so you'll need to adventure.
Several Dungeon Masters I know provide minor bonuses to those characters purchasing more extravagant lifestyles. This is a nice mechanical benefit encouraging the desire for treasure in the players. Something like +1 temporary hit point for living a wealthy lifestyle, in addition to the real world consequences noted in the players handbook.
But that's not all of what you can do with your money.
On page 186 of the new Players Handbook, you'll find a section titled "Between Adventures". It notes you must pay your lifestyle expenses, but it also notes a selection of downtime activities.
Crafting allows you to craft up to 5 gp in goods if you provide half the value in raw materials. It takes 1 day to craft something for each 5 gold of the cost. This is on top of the normal lifestyle, unless you're willing to downgrade to modest or comfortable.
Profession allows you to work a job. Super-great! *sigh* This earns you lifestyle, quite a lot if you're a performer. Recuperating is also an option that doesn't cost any extra money.
Research allows you to find information. Similar to paying a sage, but without the sage, it costs 1 gold a day over and above living expenses and takes as long as your Dungeon Master says it does to find out.
Training is the Pièce de résistance. You can learn a new language or training with a new set of tools. You Dungeon Master might allow additional training options. (!!) This takes 250 days (!!) and costs 1 gold piece per day (!!).
First, if you're running a Forgotten Realms campaign, and notice how restricted the language list is, then it is very beneficial to actually do this. If you do, of course, you're out 750 gold at comfortable. Double that if you are living large. Second, I'm fairly certain the "additional training options" will be things like proficiency in a skill or saving throw at some increased cost level.
If you look at the actual value of treasure handed out so far in published adventures, then you'll find it is quite low. Checking up and down that list you'll see that there's quite a bit to spend money on, and start to feel like you'll actually need to go out and get more, or you'll end up working for a living.
I also think it's quite nice that all these options are options that exist within the fiction. They aren't "I made a hat that gives me +2 to my brains!". They are real type activities that give a 5th edition campaign a degree of verisimilitude.