I always say that honesty is the best policy, so just explain your situation. Begin with a compliment, like "you know we enjoy your visits so much." You can then say something along the lines of needing quiet time to rest after their visit, or time to clean up the house or get some repairs done, or suggest that they have important business to attend to back at their home. Be kind, but firm in the fact that this is YOUR house, they are the guests, and their stay is over.