How can my parents stream music from an old hi-fi stack to the garden?

Heather's parents love their old hi-fi, but would like to listen to it in the garden as well. Is there an affordable way of making it, say, Bluetooth-compatible?

My parents have an old hi-fi stack system - vinyl, CD and radio. It still works great and the speakers are fab! The only thing is, they want to be able to listen to their CDs in the garden, which is quite a distance. I was thinking of getting them a Bluetooth speaker for the garden, but is there any way of making their hi-fi Bluetooth compatible? Heather

There must be a dozen ways to solve this problem, but most of them run into all sorts of ancillary issues, including cost. Is there a handy mains plug in the garden or does the solution have to run from batteries? Will the speakers be left in the garden? If so, they will have to be weatherproof. Most importantly, will your parents be happy to keep going back into the house to change CDs?