Construct two equilateral triangles on a straight line. There are two lengths that look the same - can you prove it?
Under what circumstances can you rearrange a big square to make three smaller squares?
The items in the shopping basket add and multiply to give the same amount. What could their prices be?
Take a triangular number, multiply it by 8 and add 1. What is special about your answer? Can you prove it?
Andrew Wiles, who famously proved Fermat's Last Theorem, talks about being a mathematician and dealing with being stuck.