YayCurrency plugin gives you the ability to add currency dropdown to widget sections of your theme.
To add widget, here is the step by step:
Go to 'Appearance' >> 'Widgets'
Now scroll down and go to 'Select Currency - YayCurrency' option
Choose the widget location where you want to display the currency switcher option as Widget.
Finally, click on 'Add Widget'.
Here is how it looks:
Here is how it looks at the frontend: