Not everyone is a fan of long layovers. Some airliners go the extra mile to make the layovers special for their customers. Some stopover programs are reasonably priced and some are absolutely free. I am listing a few here-
This company offers a free all-inclusive stopover program upto 7 days. You can explore Reykjavik, the beautiful capital of Iceland. It is a common stopover destination for passengers flying from America to Europe or vice-versa. You can also choose to have a local show you around the streets of Reykjavik. It is called the stopover buddy program.
2) Thai Airways