If this is a pure MR the obvious answer is the Best Western right inside the airport. Anywhere else will add taxi costs and dilute the value of your miles. Plus Best Western does earn miles in most programs. If you want to enjoy some of San Juan you will still find restaurants open and things to do, even on a Monday night. If this is your plan, the Isle Verde area is closest to the airport. The cheapest place I have stayed there, that I would visit again, is Green Isle Inn.