Since they’re not registered, I assume you’ve asked what them they want. Did they say they want nothing, or are they just being coy? People very often don’t like to volunteer gift ideas, or they just don’t know. If you give them two or three choices, though, they are more likely to volunteer a preference.
If they’re being politely coy with you, you could ask the daughter or another relative for ideas. If the couple truly has everything they need and want nothing, you can find out if you share any philanthropic causes. They might love it if you donated money to a food bank or the American Heart Association in their name, for instance. Also, you might ask her if they like to eat out, and if so, where they go when they splurge. This would be especially nice if the restaurant were in the same place where they plan to spend their honeymoon…make sure they know they have it* before* they leave, though! You can also look for the “I wouldn’t spend it on myself” angle…a gift card to their favorite music store, if they’re musicians, for instance.
As nice as it is to go the extra mile, though, a gift card at a store you know they would shop is a very practical gift…either a bed-and-bath store (because towels and sheets always wear out) or a general-purpose store like Home Depot, Lowe’s, or Sears*.
As for the cross, that qualifies as art. Be very careful about giving art for a wedding gift. The couple might think you will be expecting it to have a prominent place on their wall when you visit. The same goes for magazines…if they don’t want it, they’ll still wonder: will you be expecting to see it on the coffee table when you come by? Will you ask if they’ve read some particular article? You don’t want gifts that might make them fear being put on the spot.
Farm wedding does not mean non-Christian wedding, by the way. Catholic weddings take place in churches, but not all denominations have that constraint. I attended a Christian wedding this summer that took place on the farm of the bride’s parents. Even agnostics like to hear “what God hath joined…”
(*Sears, by the way, not only holds jobs for National Guard troops called up by the Pentagon, but makes up the difference for wages lost.)