Just to flex my Swift muscle a bit I decided to spend some time on solving problems from the Euler project. These are small mathematical puzzles you can do in your own time where you can pick up interesting mathematical knowledge. As an added benefit you learn to express your solution in the language of your choice.
You have to solve the problems in order. So as long as you did not solve problem #2 you cannot start with problem #3 let alone problem #5. And do not be afraid of running out of things to do – there are 721 problems to solve in increasing level of difficulty.
One of the interesting problems to date is smallest-multiple where you have to answer the question what the smallest multiple is of multiplying a series of natural numbers from 1 to n. Only in some cases where the length of the series is small the outcome is n! but n=4 it already deviates from that.