The rule of thumb is to keep it simple.
My go to gift is a picnic basket filled with goodies for a romantic picnic. It has varied from one that was already complete (bought online) and all I had to do was order it and have it shipped to their house, to buying a basket at Marshalls and filling it up with two of everything - silverwear, plates, nakics, etc. and even a blanket.
Another gift that I give is wine in a nice wine carrier with cork and stopper. Maybe even wine glasses... depends.
Fancy photo frames are another go to gift for many... a way for them show off their memorable time. A friend of mine gifted one of those electronic photo frames that she got on sale from Best Buy. It was a hit.