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.
We have two working students at our barn. They come twice a week, muck stalls, fill hay nets and water buckets. Then they receive a lesson and hack time.
Muck out stables for a price/ let a horse out in the morning. But only if you are reliable.
Riding out for people, tack cleaning, babysitting
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
Go dog walking, babysitting, cut the grass for neighbors
you could always do mane and tail braiding. its not very hard once you learn how...
I tacked up people's horses and also clipped them, which tends to be more pricey
When I was younger I started a tack cleaning business and charged a small amount to clean other people's tack. I put a flyer up at the barn, it was great!
Babysitting gets you a lot of money if you are allowed to do that as it isn't really a proper job
My barn allows people to work for their lessons. Every 8 hours I work, I get one free lesson. See if your barn can offer you something like that.
Have a rummage through your clothes and horse gear sort out stuff you don't wear any more and sell them on equestrian Facebook pages