This is a special gift, so perhaps you could give a budget guide so as to get the best advice. In the meantime might I suggest, a weekend/week away for the happy couple. Somewhere that would be relaxing and interesting to them.
I did think of a weekend away but they go away alot and so it's probably not as much as a treat as it would be for others. It's my back up plan if I can't think of anything else! Budget wise I guess I'd spend around €400