Why?

The original first century Christian Symbol was a fish with a stick figure walking out of it's mouth. It was called the Sign of Jonah. It represented a spiritual rebirth. Notice that it wasn't a man walking out of a cave like Jesus. That would have been misinterpreted, and it was ultimately.

A cross, inside a circle, with a diagonal line through it.

Darwin name in a fish outline? Ichthus spelled in Gk. in the same?

Be Careful because enemies often enough end very similar!

Interestingly enough there is historical precedent for this, the pagan Vikings never wore Thor's hammer until after they encountered Christians and saw of them wearing the cross, quite literally they liked the style but did not wish to wear the symbol of another religion, as a result of which they reached into their own mythology and produced pendants of Thor's hammer. You could for example acquire a charm, such as might be found on a charm bracelet, of something which represents science, for example a microscope charm, which you can wear as a pendant, symbolising reason over superstition.

Plenty of them.

As one of us godless heathens, you would have to choose which you prefer.

No god to tell you now.

Buddhism is rife with symbols, especially at the Tibetan end of the spectrum. Wear an OM?

The celts seem to like the tree of life.

Wear a candle for amnesty international?

Heck, your local sport team could do with one more cheer.

On the other side of one of the religions?

It's your choice as to the symbols you use.

The cross is a fine and powerful one. So is the OM.

Empty Set

I have a religious symbol. It's a question mark.

I wear the "atheist A" its a nice sterling silver pendant, not big and on a sterling chain.

Casper the Friendly Ghost? Sorry, don't mean to be snarky, but as an atheist I'm not a big fan of the "supernatural" at all. & "spiritual" seems not only wishy-washy, but kind of a cop-out, as well. Do we all have more than just a material basis? Probably. But the term "spiritual" carries about as much baggage as "theist", & can mean just as much or as little as the individual wants to put in it. What type of "spiritual"? Pantheist? "New Age"? Ethereal beings? Buddhist or Taoist? Pagan? All may have their own type of "symbol". Maybe a yin/yang symbol?

The plaque

[amazon.com]

I'd go with this. There are also plaques to put on your car.

[etsy.com]