The idea is to be creative and get them something they can treasure for a long long time. If you cant,then you might as well get them the vouchers and the gifts.
One other gift that you can give is getting a favorite photo of the two of them and have it drawn by an expert artist - especially if they appreciate art . then have if artistically framed - this will cost you alot and needs time for the work to be done...... so if you intend to give them like tomorrow,this is too late already.
this is also a gift you can give to your parents,siblings,friends,spouses etc too for other occasions.
Experience is what you get when you don't get what you want.