That's nice of you to be so thoughtful :)
I've been through lots of infertility crap, and just a thought on some of the previous suggestions, wine, or alcohol gifts would have been a kick in the teeth for me, so maybe tread carefully with that! To me, alcohol gifts said "here's some wine, because you're not pregnant so you can still drink it". Similarly, wineglasses, wine racks etc.
But you'll know them and whether they would have the same response as I did! I know it was a silly response really, as people meant well, but all I could see was "well you're just not pregnant".
My favourite gift when we were in the throws of it (and it really takes its toll on your relationship), was a selection of fun and silly little gifts, all wrapped and numbered. We were also given a jar full of slips of paper, with a number on each slip. The idea was, when we had a few evening, we'd pull a number from the jar, open that gift, and do whatever the activity was inside. I can't remember them all, but there were things like mini canvases and a paint set and we had a painting evening (both crap at art., but it was quite hilarious!), a little kit to bake and decorate cookies together, a DVD etc etc. So just lots of fun things to do to enjoy just bring worth each other, not thinking about the problems of infertility.
There are probably much easier ideas out there though! Experience gifts, as mentioned above, sound good also :)