Assuming both are in agreement, I would start by thinking of your closer male friends - ones you know and trust and who you know are into this sort of thing. A lot of danger with strangers involved. At the very least, you have to be careful to get to know anyone new properly before you start. Both you and your husband should probably meet this person in advance as well. I know that this risks spoiling the fantasy a little but you can get round that by keeping some aspects of what you have in mind secret such as when you will do it and even exactly how far you will go. If, for example, you are in to poly you can introduce him as a potential partner who you will see when he is not around. If you are mono in normal circumstances (to the extent that you trotting out a man in front of him will cause problems) then introduce him as a lifestyle mentor. Just make sure that hubby gets along ok with this guy.

To make it a real surpise, you could start things off romantically - you seduce him, tie him up, as far as he is concerned you are alone in the house. Then, at a preset time, the front door bell rings and you leave him in the bedroom to go and answer it (or you open the bathroom door to reveal your lover standing there, whichever is easiest). He's there, all tied up, and sees this man come in and has no idea what is about to happen....