It's become more and more commonplace for people to want money i lieu of gifts for many reasons, but it's still not considered proper etiquette to say so outright. This doesn't mean that people won't/don't in their own little ways. One popular way is to let guest know that they will have a Wishing Well, click here to see invites for a wishing well. You could also put on the invitation "monetary gifts preferred" but that is considered tacky. If you know most of your parents friends, then you can let them know if they ask that they really have everything (that might give them a hint). Another way is to let everyone know that they are saving up to go on a 50th Anniversary trip or second honeymoon.
My apologies, the wishing well link is here. You could use one of the wishing well poems to let them know about the trip, or telling people mouth to mouth is the next best thing.
I welcome your thoughts, let me know if you want to talk more
It's not hard to change anyone you like to suit your purpose, for example, here's number 9 with changes
Our home is quite complete now, we've been together so long, so please consider our request and do not take us wrong. A delicate request it is, we hope you understand. We're going on our second honeymoon maybe you can lend a hand.The tradition of the wishing well is one that's known by all. Go to the well, toss in a coin and as the coin does fall, Make a wish upon that coin and careful as you do. Cause as the well's tradition goes, your wishes will come true.So on this special day of ours, the day that we'll be wed. Don't hunt for special gifts but give money in it's stead. And as you drop the envelope with money great and small, Remember, make your wish as you watch your money fall.
"If you were thinking of giving a gift, to help us on our way.A gift of cash towards our 2nd honeymoon, would really make our day.However, if you prefer to purchase a gift, feel free to surprise us in your own way."
it tells them what you want but mentions that they are free to do as they wish.