I just joined mamasource today and if 50 is a little odd, I probably won't be around long, because I turn 50 this Fall. For you though, please ASK her what she wants, unless you're hell bent on a suprise, which personally I feel isn't necessary. Fifty is such a sensitive time. You go from being called momma to mother in law, and you aren't really sure how it happened. Make sure that everything you do for her reminds her of how cherished she is in your family and what an impact she has made on you. There is no going overboard with the sentimentalities here because at 50 you really need to know that you are still vital in your family even though you aren't running to soccer games and swim meets anymore. I always tell my husband that any gift he gives me must pass this test, Does it make me feel beautiful? Does it make me feel loved? If so, the gift is a winner.
Be sensitive about red hats. Not everyone wants to join a bunch of gaudy women who parade around town wearing a hat that tells everyone of their age. ASK! I for one, if given a red hat on my 50th birthday will probably sit down and cry.
Good luck to you, I know you will show your love for your mother-in-law in appropriate ways that will make her feel loved and cherished.
Ps. After reading the responses here I found it odd to think of a 50's theme for the party. Then I realized I was born in 1958! (Duh) Most of us born in '58 really don't identify with the 50's because we don't REMEMBER them at all! On the other hand, 60's would be an era I would identify with more.