Hi Princton Mum,
 
I think for someone that age, one of the most lucrative and flexible options is babysitting. She can choose when to work and she'll probably have no trouble getting hours on Saturdays and holidays.
 
To help her get started, you could look for a Red Cross babysitting course being offered in your area. That will teach her the basics of child care and first aid and will make hiring moms feel much more comfortable leaving their kids with her.
 
Then I'd just let neighbors and friends who have kids know that she's available. You could also post a notice in the children's room of your local library but obviously, if she ever sits for someone you don't know you'll want to go with her to the interview so you can "screen" the family and home at the same time that they're screening your daughter.
 
Good luck!

When I was that age, I worked seasonally in food service and retail. It's easier to find work during the holidays than on weekends, but I really think that food service was the perfect job: it was fun and social, taught important lessons about customer service, hustle, and how doing it right the first time will save lots of effort in the long run. These jobs can be hard to find for teens, however. If you have any family friends or her friends' parents are involved with any restaurants/cafes or retail stores. She will also need to get a work permit to do these sorts of jobs.

In addition to babysitting and retail jobs, I also worked as a florist assistant (and floral designer) when I was in high school (after obtaining a worker's permit). Weekends and holidays are busy times for florists, and they often need temporary or holiday workers -- even if they don't have regular part-time positions available. I found this type of work to be very fun: being surrounded by the smell of flowers, meeting customers celebrating various occasions with flowers, socializing with creative co-workers, and learning this trade.
 
Good luck to your daughter in her search for a job!