In my games, at the end of every adventure, there's a "night market" where the magic item smugglers hold an auction. I have a few items up for auction, and the PCs bid on what they want. Because there are other adventurers bidding, you can usually see the PCs lose a HUGE amount of money for items they want. Last time, we had a bard spend something like three times the listed price for a magical bardic instrument.
In the same markets, I also list several interesting, but not necessarily powerful, treasures. For example, in the last one, PCs bid on a rock that teaches you the language of earth elementals. The eladrin wizard won a petrified pseudodragon (which he hasn't been able to turn to flesh as of yet). Lost maps for dungeons were up for sale, as well as plots of land. In the next market, I plan on selling a black market dragon egg (species unknown), as well as young exotic mounts. Plus some cool foods, objects d'art (for the PCs' keep), and unidentified magical potions. All for auction, of course.
Finally, why not give them access to services? If they know an assassin who will remove foes for 5K a pop, some players will take advantage. Unusual one-use items are always fun to bid on, but so are weird toys... things like shield guardians, smaller constructs, and weird mysteries.
Having PCs spend on training is a great idea, as is having them spend money on henchmen and hirelings. If you make building a home a part of the game, and drum up a few mechanics (Paizo's Ultimate Campaign is a great starting point), you'd be amazed at how quickly and readily your players will spend money. I'd not be surprised to see them spend twice as much money for a spiral staircase as opposed to a normal one, if the option is in front of them. After all, a lot of people want to build their own home in real life, but lack the funds to do so. Give them an option, and they'll spend all sorts of money to realize their dreams in a fictional space. This is why paper doll computer games work so well!
What about pets? Dogs and horses and the like are simple. But what if the PCs purchase an elven dog for 1.5K? Or a baby drakeling? Or a faerie dragon egg? Not all pets need to grant combat advantages, of course. Some could just help keep an eye on the camp while the PCs sleep.
Diviners, fortune-tellers, and the like are another option. PCs could get their fortunes read. Imagine if they visit an important oracle, and she tells the PC of a dark future. For 2 or 3K, you could have the player roll, say, 4d20, and record the results. That PC can draw upon those rolls in much the same way as a diviner character. A lot of players would love that!
You could sell all sorts of class abilities this way. One use potions that replicate a barbarian's rage. For a LOT of money, you could sell a monk's first level unarmed combat damage as a permanent ability. A non-rogue could spend 5K to learn thieves' cant, or druidic. You could sell training for weapon proficiencies, armour proficiencies, or tool proficiencies. Instead of 250 GP over 250 days, imagine 3,000 GP for a 30-day training course in how to use masonry tools (or whatever item your players have their eyes on).
As a really fun one, you could have an NPC with a random item, such as a Deck of Many Things. He charges people three or four thousand GP, per card drawn.... no refunds. Have fun with this one... and watch your more risk-taking players happily destroy themselves.
Likewise, spending money on cool-looking arms and armour is an option. High class clothing is the same thing. Have PCs be obligated to throw a few high falutin' parties. Or, if they get themselves an expensive campaign manager, they could run for office!
Having a lot of money, and NOT being required to spend it on items in a never-ending progress quest, is a good thing. Open the game up, drop a few hints if your players are stalled for ideas, and I guarantee - they'll find things to spend cash on.