Xmas experiment query- cancel idea!

Has any one suggestions about the best way to drive 2 strings of Poundshop battery operated lights.
Each string has 10 Leds connected to 2AA battery holders. I would like to alternate them lighting one string and then the other. .Needs to be quick simple and cheap to do. Too late to order any hats for this xmas.

What components do you have handy to drive them with? A simple transistor should suffice to switch them on either the supply or ground side, with any Pi IO pin, or micro-controller for smoother transitions.

Nothing yet, only had the wife ask at tea time if it could be done.
Raspberry Pi Zero is only part of the equation at present.
The only transistors in my current collection are RF power output units and waiting to be fitted into a 2m (144Mhz) amplifier