By the law, you are committed to the lease; prepaying has nothing to do with that. So yeah, you can store stuff in there, or perhaps investigate doing AirBnB.
I assume you'd rather have the money back, though.
You are allowed to "break the lease" by mutual consent with the landlord, because both of you are allowed to agree to change the terms. This is done all the time. The landlord "moves up" his efforts to find another tenant, and once the new tenant starts paying and moves in, you are off the hook - the lease is broken.
Normally, you have not prepaid the rent, and the landlord will naturally not want to bother finding a new tenant, and just keep dunning you for the rent. In this case there is a legal concept called "mitigation of damages". When you are in a contract and suffering financial loss, you must do what is reasonable to prevent that loss, you can't just sit on your hands and make the other party eat the loss. Breaking a residential lease is the most commonly used example of this legal principle. So the landlord can't get a court judgment for back rent owed*, unless he can prove he really did try to find a new tenant, and all were unfit.
However, since you prepaid, that is a new wrinkle. He's not the injured party: does he still have the duty to mitigate when it's not his loss? I simply don't know what the law will say about that. I do know it'll be prohibitively expensive to find out, unless you can keep the matter in small claims court. On the other hand, the landlord doesn't know how that’ll go, either, taking it to big court is mutually assured destruction.
So practically, you are at the mercy of the landlord, and your best bet is to ask, cajole or threaten him to mitigate damages by finding a new tenant, and refund your unused rent once he has done so. It's fair for him to charge you some costs, which you can mitigate by helping promote the unit. As soon as he puts up a listing, buzz it on social media etc.
Don't be shocked if he is unable to mitigate all the damages, due to tenants that don't pay etc. Landlording is a tougher business than most tenants realize.
Refs: reply #4 here, landlord's legal advice was he was obliged to seek a new tenant to relieve the departing tenant of obligation. Another landlord advised the same.