I started volunteering in one last month (RSPCA) and I'm enjoying it.
It's a very relaxed environment and I get on well with the other volunteers and employees. So far I've worked on the till, organised donations, arranged items on the shop floor and greeted customers.
Plus you get to know the time you're putting in is for a good cause, as well as being good for experience.
I volunteered in a Trussell Trust shop for my Bronze D of E volunteering when I was in year 10. I volunteered for one hour per week. I mainly sorted donations and it was really eye opening to see what people donated. Some people would donate clothes that had been washed and folded. Others would donate clothes that were dirty/stained and would just bung them in a bag!