Here are a couple of ideas you might not have thought of.
1. A himilayan salt lamp, even the most stubborn scientific brain can feel the energy shift.
2. Something to help an ailment they have, maybe an excersize machine or a latest gadget or similar.
3. Gemstones and crystals can be great, then you can either choose one in the correct stone for the month, the anniversary or each of their birthstones.
4. Books on a favourite topic.
5. Its amazing what handmade items you can find on etsy.
6. Folded book with their wedding date. If you have the money you could get 2 books, each with their names.