hey I am surprised to hear that as the daughter in law you will be responsible for planning the fun. I am a newly married and to be honest, I am expected to fit in with my in laws, the only time i was encouraged to take iniative is when it comes to housework!!
Also just want to say to the original poster that I would be highly weary of the notion that you may slowly be able to inject some fun. Chances are that they would expect you to fit in with thier way of life. If you try to encourage more "fun activities" they may percieve you as using these events to take away thier son, esp if they are not always able to come along etc. This could open up a whole van of worms. Thier lifestyle has been built over several years, even if you can change a person but how can you change a whole family.
anyways thats just my two pence.