Hiya!
If your Player's Character's are walking around with tens of thousands of GP's (in various forms) and "don't know what to spend them on", well, imnsho the problem isn't that THEY don't know what to spend them on...but YOU (the DM) don't know what to spend them on.
PC's stagger back to town, wounded, depleted of equipment, and ready for a rest...
Guards: "Ho ho...! Looks like adventurers have returned...barely! Congratulations! You look like you all need a good sleep and some food and wine in your bellies, no? Well, we'll make this quick then. Standard adventuring treasure recovery tax is a nice, flat 20% here in our fine, prosperous country! Be glad you didn't go to Yonderville, it's in Uthercountry...they're on rough times...heard their adventuring treasure recovery tax is at 50%! Heh...probably know that, huh? Probably why you came here. Anyway...lets get this done. Gotta keep the tax man happy, right?"
..
PC's get to the Golden Horn Inn:
Proprietor: "My Stars! You folks look downright disheveled! Here, let us get you the finest rooms...in fact, it's an entire floor, all to your selves. Four bedrooms, a spare one you can use as another bedroom or for storing gear, two private bathrooms, full hot-bath services and laundry, food, two sitting rooms and a large room with a large fireplace...even get a healer to come help you guys if you want. No no no....don't worry about the price! I'm sure you fine adventurers can easily pay the meager fees! You won't regret it! Finest service in the city!"
..
PC's are having Brunch at Golden Horn Inn after a nice nights rest:
Proprietor: "Good sir's and madam's, sorry to intrude, but I have been inundated with messages from some of your...friends? Favoured shopkeeps and artisans? Here's the list of them...14 to be exact! Busy...and popular you are! And don't worry about paying for brunch. I've managed to get some good deals on that Sweet-Honey-Wine that Mistress Beatrice was sipping on last night! You staying here has brought in new customers, for sure! In fact, I have two free bottles of that Sweet-Honey-Wine for you from the local winery...'Teebles Brews and Distilleries'...best alcoholic craftsman...er...crafts-halfling...in the country I'd bet! ...I believe he's number 3 on your list there..."
..
...and so, the PC's pay taxes, pay for room and board, get requests from other patrons/businesses offering "deals" on their wares, and I'd fully expect various religious people to come a'knocking, as well as artists, bards, torchbearers, porters, etc...etc. The PC's should have no trouble at all finding things to spend their money on....because others will help them. (or at least try!)
If your Players are anything like mine, they'll be out of coin in a matter of days. Spending it on food, wine, parties, new equipment, hirelings, fancy new clothes, getting their various pieces of equipment "fancified" (coated in silver, gilded, gem-encrusted, carved/engraved, etc). Oh, sure, they have have a few thousand left, but that won't last either.
If you want to really make your Players "get into" the world...you need to provide them the means to, uh, 'invest' in it. The best way to do that, in my experience, is to offer them opportunities to spend it on 'the world' in stead of a boring new magic item. You can always find a +2 Dagger in a dungeon...you can't find a good carpenter who is willing to help build your new house for 25% off!

The dagger won't help develop "ties" to your campaign setting or it's NPC's...that potential friendship with a good carpenter will. And, IMVNSHO, the later is
infinitely more desirable than the former!
(in other words, "Ye Old Magic Shoppe" is a cop-out if you ask me...which is why there are almost none in any of my campaigns...ever...).
^_^
Paul L. Ming