Aren't quiet pubs/bars for that? Or say you've had a busy week but how about takeaway and some trashy TV in the background? Been meaning to try some local restaurant, you guys want to see if it's any good?
Going out also costs a chunk of change, and snarfing a grocery pizza will cost a lot less. :-)
Another option to frame it casually is say "X and I were going to get Thai and play some Fluxx. Seriously, nothing elaborate. You're welcome to join us if you want?"