An example of sayings for 50th wedding anniversary cards is, "Anyone can fall in love, but not everyone can stay in love. Happy Anniversary! and here's to many more." The writer's relationship with the couple celebrating the anniversary plays a large part in what to write. If the writer has a professional or more somber relationship, then a quote or general well wishes may be in order, whereas someone who is a close friend may write a funny quote or joke.
Funny things can be tasteful and kind, without being rude, mean or vulgar. An example of something funny or cute to write in an anniversary card is, "50 years ago two people got hooked and today a golden recipe they have cooked," followed by whatever closing the writer would like to use.
Most quotes are written at the end of a personal note. Other options include, "Love is what you have been through with someone" by James Thurber, "Love is the expansion of two natures in such a fashion that each include as the other, each is enriched by the other" by Felix Adler and "Love has no age as it is always renewing itself" by Blaise Pascal.
General well wishes can be anything from congratulations to best wishes on an important occasion. Sometimes something simple can be just as nice as a quote or a funny saying.