To maintain interest over time here, I let the children negotiate the reward system for each activity.
As they get older the rewards can be spaced out more but they tend to be more substantial.
A time will come when they don’t need a sticker every time they play piano but will take one sticker on a chart at the end of a weeks practice and them trade the full chart for something bigger.
At the moment I have one child earning Lego by doing daily piano practice. One earning a 2 barbie movies for completing a math text book and one earning an iPod for completing Saxon 5/4.
I very rarely have more than one incentive going on at once. I do think my kids are old enough to know some things are expected of them. so i only reward things that other “normal” children wouldn’t be expected to do. I am happy to reward with goods and toys as I don’t tend to buy them things outside of their birthdays or Christmas.
For pocket money…well I confess I am not good at this, who has cash these days? lol the girls are currently doing extra chores to earn money to send a shoebox of toys to a child in need for Christmas. It should be interesting when we go shopping for this one! It was their idea so they are going to have to follow throu and hand over the toys! 
Here’s another reward idea. Take a jar, and every time a child does something reward-worthy, put a bean in the jar. This can be a family project, to teach the children that we are a team, and that everyone benefits from each member’s success. This can be a reward for chores or academics, being polite, or even effort for overcoming a child’s shortfalls, such as NOT having a meltdown over being interrupted. (struggling with that one right now). Anyway, once the jar is full, the family gets to do a fun activity together. Some families pair the fun activity with a service project, such as visiting a rest home and then going to a restaurant with a soft-play. The reward might be going to a museum, or the zoo, or buying a pizza and renting a movie. Whatever works.