simply let the domain expire
Are there any formalities to complete or should I simply let the domain expire?
One other alternative might be to sell the whole site and domain.
That's assuming it's currently only costing you $10 a year or so and the name doesn't have any particular value to others. (I mean, ahem, actual value.
I'll keep the sites for now.
But if you are truly done and it is time for something else in your life, all of the suggestions above are excellent.