There was a recent story in The New York Times on tattoos and one of the tattoo artists interviewed gave the following advice (I’m paraphrasing):
If you go into a tattoo parlor and ask the artist what should you get, don’t get anything.
The same goes for jewelry engraving.
You don’t have to have your rings engraved but if you want to do some research and find something that is meaningful to you and your fiance go ahead. You don't have have a lot of room so keep it simple. Some timeless ideas are your names, initials, married monogram and/or wedding date. Ask your jeweler how much space you have, and remember that character count means the limit of space you have using any combination of letters, numbers, symbols and spaces.