I think you’re misunderstanding both a wedding and a gift. A wedding is a celebration of their marriage, not a pay as you go event. And a gift is something willingly given, not demanded, not enlisted, and not required.
You cannot even properly say that you want cash only instead of a gift.
And they better have that honeymoon paid for before the wedding because reservations are required at most hotels.
If you collect money at the wedding in a box or basket then you’ll also need someone trustworthy to keep it under constant view.
Honest, you don’t ask for money, and so there is no best way to do it. If you’re going to do it you’ll do it, but it’s not part of anything ‘best’.