A unique centerpiece item would be a small beta fish bowl with a colorful fish in each. The bowl can have just a simple array of stones at the bottom and makes for a fun & long lasting memory for the day.
You can always go classy with flowers, or festive with balloons. If you want it to be funny, you can creating huge hills (some snow covered, some grassy, some sandy) with a person up and over it.
Usually people associate turning 50 years old by having them gone over the hill in a sort of speak. There are really nifty "over the hill" accessories at Spencers gifts in the mall. Another idea can be using something that the person idolizes or what their hobbies are and incorporate that into the centerpiece.
It depends if it's for a man or woman, if it's supposed to be a classy affair or a funny theme. i think you could do something with hills and then an old fart looking person on their way down the hill, as if they are proverbially "over the hill".
