Congrats on your find. Have fun!
Will you and your husband be the only guests at these people's house? If so, I say leave the knitting at home--unless you're really good at knitting while looking at other people and talking. If someone came to my house (even someone I don't know well) and pulled out something else to do while we were visiting, I would consider that rude. Of course, I consider it rude when people I'm visiting watch TV rather than turn it off and talk. That tells me they don't really want to speak to me all that much.
And building up a stash is often not a very "inexpensive" thing to do--especially if you're looking for some of the higher quality yarns. There's surfing the Internet for sales, looking on eBay and Ravelry, and looking at thrifty stores and garage sales. But while you may find some cheap yarn in this way, it's probably not going to be "fast."
Why not focus on getting the yarn you need for the next project?
Many of us have stashes, but we're not particularly proud of them. At some point--like, when you realize you could and should be using your stash for the rest of your life, and yet you don't really have the perfect yarn for that next project--well, you come to understand that stashes aren't all that great. Yeah, it's nice to go pull out some yarn and start knitting immediately, but it takes up space and is a waste of money if you don't eventually use it.
Besides, you'll build up a stash soon enough because you'll have leftover yarns from all the projects you do.