Currency is not really needed.
One game comes to mind. Immortal Cities: Children of the Nile. There is no 'currency' as far as coin or anything like that, everything comes down to food and crafted goods. Who would have thought that an economy revolving around bread and vegetables would work.
Another thing would be World War II.
Different military units would have to deal with not just enemy blocking supply routes, but of red tape blocking supplies. Just about anything was swapped for just about anything else.
As a rough example.
One unit might have secured extra fuel, but be in bad need of spare parts, so they trades some of the fuel to another unit that needs fuel, a that has some extra cases of coffee. The first unit then takes some of those cases of coffee, and trades it to another group that is out of such 'luxuries', but that happens to have spare parts for vehicles they don't have, but which the first unit does have.
More often than not, no money exchanges hands, just goods, supplies and favors.
Instituting a money based economy on something like a Mars colony seems like like a bad idea, if only in concept of what kind of havoc that greed could reek on something like an 'isolated' Mars colony. Although, in a way, it also opens up other opportunities to undo such issues by taking a 'more time and effort intensive' act of charity/generosity to counter it.
There should also be a sense of teamwork going on.
Of the main base occasionally sending requests for items, materials and supplies intended for use in helping to establish further outposts, support surveying/prospecting missions, or even work on a much larger 'base' meant to be the first 'real' colony on Mars, meant to house not just some brave pioneers, but to be capable of handling full blown families, for 'living and thriving' on Mars, not just 'surviving'.
Something to give a sense of something beyond getting the best score, or the biggest trade value, or getting to the top of some leader board, but of accomplishing something great...
Although it also pains me to bring up the subject of another 'motivator and goal'. Challenges and Achievements, which a friend of mine will obsess over in games...
One other stray idea for a sandbox mode, perks. Have a few 'points' based on rank, to select for a base, or the group as whole, or just for themselves. Maybe belt mounted batteries to extend EVA power supplies, or something that makes Rovers more fuel efficient, possibly with a small loss of top speed, or something that allows the game host to have the base start with something like an extra case of MREs or medicine.
And ideas that will likely make you simply go 'um... no...'
My bug list is 'small' compared my list of things that might be neat or interesting to have in MC:C. That is, bugs take up 1/4 of the text file I made for MC:C stuff, the rest is 'ideas'... which I will probably 'wall of text carpet bomb' into a topic sooner or later... after I 'beat' the game... still in Phase 6...
If mining is Ahab, then Lithium bearing ore is Moby Dick... despite having work orders done... to heck with the score, I want repair kit materials, just in case... better to thrive than survive. Or something like that... but back closer to the topic of sandbox modes and such...
I've done basic modding for games like Elder Scrolls III Morrowind, and a little for ES IV Oblivion, and Fallout 3. Just looking at all the stuff in the files boggles my mind, even with the 'construction kits' they come with making it 'easier' to look through the files... I am surprised the people at Bethesda Softworks have not gone insane trying to keep track of so many variables and such with just making one of those games, and am not surprised such games often have tons of bugs in them... although I have to basically 'face palm' when the modding communities tend to crank out unofficial patches for bug fixing far faster and more effectively than the developers seem to do.
I guess what some people say is true. When you love your job, you will never work a day in your life.