Muck stalls, offer to handwalk horses, clean tack... I'll even pay somebody to go and groom! Usually horse people are very happy to help find a job for a young person looking to make some extra cash and horse time.
Offer tack cleanings for other people at the barn? I would definitely pay €5 a week if someone cleaned and polished my tack weekly. Even if they used my tack cleaning stuff. Just make sure you do a good job! At least in the beginning