The spa certificate works well with the younger crowd.....like in their twenties because this is often something they can't or won't afford for themselves. Most women in their late 30s and early 40s have already enjoyed luxuries like exclusive spas...in fact these have likely become a part of their routine. Plus....spas are fun when you do it as a couple and that might become a bit pricey for a gift.....
Unless your a housewife, I have yet to meet a working woman who is in her 30s-40s and is a spa regular. Most young people are not able to afford it and when they grow older, other things become more imp then yourself. This is the perfect time to get away from family and job and have time all to yourself!