assuming you are in the USA, i would recommend making this a gift. you can gift someone up to 14k$ per year without paying a gift tax. additionally, you can gift someone up to 5430k$, but it is a one-time deal (tied to inheritance tax).
you can have a general expectation with the giftee that they will return the money with some interest on some schedule. you can even write that payment schedule down. but for legal, tax and personal relationship reasons, you should both consider all these payments gifts. if you think failure to repay will put a strain on your relationship, then you shouldn't make the loan. if not, then the payments in both directions really are gifts.
as a gift, legal documentation can be really minimal. really, only the gifter has to document anything, and that is just on their tax return. even then, i think it is only necessary if you exceed the 14k$ annual gift limit. i would however recommend documenting any interest rates or payment schedules for personal reasons. being a gift, these numbers are just guidelines, but at least you won't find yourself arguing about whether or not the guidelines are being met.
depending on how large the amounts involved are, you might want to spread the load over several people to stay under that annual limit. but the limit is per tax year, so you could do half now and half in january.