First off, can you get away from the family? If so, you have to go for a long run. A really long run. Use the excuse "You got lost". Don't carry your phone, but make sure to bring cash. Find a local bar/restaurant and hang out there for a while.
Second, see if the wife wants to join you in an hours long excursion (depending on where you are). Go out and explore local parks (if you haven't already) or if you have done that, find further ones to explore.
If you are near a city and have done almost everything already, tell her you want to go back to one of the places. Even if you don't really. She will either say "sure" or "you can go by yourself". If she says you can go by self, make sure you ask her 5x if it's okay and that you don't really want to because you feel bad for not spending time with her family. You come out looking like the good guy and she probably won't even think twice about the fact that you really want to avoid the in-laws.