I think buying someone experiences is always better in the long run than buying them 'stuff'.
I took my hubby away for a couple of days to an eco cottage
http://www.hairyhobbitcottage.co.nz/hhc/Photos.htmlwe don't get much time off together and it was fantastic, really different from our other holidays - LOVED the outdoor bath, thehammock, fruit & nut treesand all the native birds!
I bought my parents and grandparents a half day sail on a 107yo 3 masted sail boat. We're a sailing family, we know the harbour we sailed like our own backyards but seeing it on such a gorgeous vessel and being able to sail something so different was so special they still talk about it!! It's alsothe only boat of it's kind around here and whenever it was out somebody would point it out so it was quite special to get to sail it.
My mum was bought a helicopter ride for her 50th - they got to land on a glacier which made it even more memorable for her.
My grandfather was bought a parasail for his 60th and a Glider flight for his 70-something.
I'd love to buy someone a hot air balloon ride... not in the least so I had a good excuse to go myself! I'd buy hubby a skydive, but i don't know that I'd be brave enough to do it myself.
So there are my suggestions: a holiday or an adventure sport!!