I can tell ya several things that would be very valuable in the real world and possibly a fantasy one in lieu of coin. To reiterate some of what the others said and to add a few.
Animals, land, tools, certain professions and their labor (blacksmiths), certain goods (paper), valuable but hard to come by goods and commodities, slaves (in certain settings), indentured servants, heirlooms and legacies, raw materials, food, water, survival stuffs, even appealing but basically useless items like beads (if they appealed to the natives or players because they were rare or exotic).
But my suggestion is this. You might be overlooking the player's creativity in exploiting the environment they are exploring if you're thinking about pre-conceived treasure handed out by the DM as the only possibility to their advancement and real fortune.
If it were me, and I was exploring land, and could get my government to support me giving away a few basic but useful or even useless exotic goods in exchange for buying raw materials from the natives then I'd start a business or enterprise.
I'd buy trees, lumber, mines, water rights, settlements, land, animals, etc. and pay trusted associates (I shipped in) local peoples who were good at business (and that I had trained) to run the enterprises for me. Then I'd ship the goods back home and take the money. With the money I'd furnish and equip my own future expeditions, the raw goods and profits from my business ventures and enterprises being my treasure (and my growing treasury). And being reinvested in future and expanded enterprises.
As my business ventures expanded I'd introduce into the areas I was exploring new goods, like paper, refined metals, decorative wares, etc and use these goods to purchase or stake more of the lands and materials I explored. Then your income stream would flow in both directions and would be immensely profitable. (I've done this before as a player to finance player expeditions, to build Keeps and libraries - for my Wizard, to fund public works, to buy political influence, to build churches and guilds, to equip myself and my party, to do charity and philanthropy, and to gain an enhanced PC and player reputation. Also once I bought and equipped a ship and formed a small private militia as a Frontier's Guard. I being the Lord of the Guard after the local King made me a Knight and eventually a Duke.)
Then I'd pay off my initial investors or government and be entirely self-financing. At the same time I'd establish good relations with the locals and build them up politically and militarily and establish my own Realm (semi-independent, entirely independent, or at least operated by a generous charter). My point is don't let your money sit idle and encourage the players to look at their treasure not as a static thing, but as an active, living thing that can be employed in all kinds of profitable ways.
In this way you could establish your own kingdom on the basis of your previous trading and business enterprises (while taking profit and building a self-sustaining financial base) or at least make a deal to form your own Frontier Buffer Zone which you could probably run as you wished (that is mostly independently). My one word of warning, whoever gets good at this will attract the attention of good men who want to join your success, but also mere hangers-on, and will cause envy in those who will want to take by force what you have built by craft. But that's a good player storyline too. Having to defend against schemers your hard won Realm.
Now your players might not think all of this out at first, but I wouldn't necessarily assume they couldn't, and if you encourage them, then they might just play this out on their own, or a very similar scenario. In that way you don't need to give them treasure so much as allow the opportunities for them to develop their own methods of independent Wealth Generation.
Good luck with it, but in my opinion, just supply them with the raw goods and opportunities and then sit back and let them see how they will actually exploit their opportunities. If they're smart they can build their own Realms, and start their own ventures and enterprises and capitalize on their own industry and imagination.
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