I use a cash-for-EXP system in my Doom & Tea Parties game currently. Spend a single gold piece on anything except hiring hirelings, anything at all, and you get 1 experience point.
Getting to the mid levels means dropping some serious coinage. In the Cook Expert book, a fighter needs 16,000 EXP to achieve 5th level. That's a series chunk of change, and the fighter's on the low end of the scale.
This has created a situation where my players actively look for opportunities to spend money. Equipment for hirelings, extra rations, identifying treasure, treasure maps, buying a round for everyone at the Oarsman's Rest, it's all fair game. However, it can be a challenge at times, so here follows some more practical options for spending money:
some potions available on the open market. Healing potions are pretty important to the current crew since they don't have a cleric among them. It is possible to commission the crafting of a potion not generally available in Pitsh, but the price is considerable, generally clocking in around 200 gp for the simplest to make.
It's sometimes possible for sorcerers, elves, and pixies to buy new spells, though its extremely rare for these to be available. And most would rather trade spells for spells.
The PCs already own a ship and rent an apartment. Generally speaking, every year, they can spend 10% of the original purchase cost of any piece of property to maintain it at its peak condition (in the case of ships, this means scraping the hull of barnacles, replacing the rigging and sails, and stuff like that). This will generally take a week's time for every 200 gp spent.
The same can be done for mundane personal gear like clothing, armour, weapons, ropes, and the like, but in general it's easier to replace rather than repair. Most magical gear doesn't require this sort of upkeep, though individual pieces may have their own special needs.
As time goes on and the PCs become more (in)famous, they'll need to worry about their property in town while they are away. All sorts of protections are available, from the mundane (better locks and bars on the windows) to the magical. Guards and guardian monsters are also possibilities.
The PCs have already made something of a splash in town as good employers (they only ever so rarely lose a hireling on an adventure these days). Spending coin to attract new employees (rowers and sailors for their ship as well as retainers) attracts attention and gets you talked about. But it's entirely possible to take this to the next level.
Pitsh is a new city, growing on the ruins of an older one. As it grows, it requires all manner of public works, from expanding the city walls to paving the streets. Keeping the sewers clear is vital as the city exists in a tropical zone and sees rain almost daily. Various services, from the magical to the mundane, are required to keep disease from spreading through the population.
More focused gifts and donations can result in improving a character's relationship with any particular group. The three temples to the gods within in the city and the temple of Tiamat outside the city will all gladly accept donations from individuals (though at this time none of them are hurting financially). There's a loose association of merchants and ship captains in town, as well as other trade groups, some fairly ad hoc (the farming community outside the walls, for instance) and some more organized (the Guild of Non-affiliated Scribes, for instance).
Spending coin on presents, fancy meals, fine clothing, minstrels, etc. in amorous pursuits can chew through coffers fairly quickly. Maintaining a mistress (all the PCs, though not all the players, are currently male) can be even more expensive. Of course, if you can't win love, you can always buy it...
They cost coin to purchase, coin to house, and coin to feed. However, they can do a lot for a PC, including spending coin doing and managing the options mentioned already while the PCs are away. Of course, these sorts of things can also be done by hiring free folk, but they don't come with the guarantees of loyalty that are the hallmark of the stock of the Shkeenites.