Funnily enough up and down both work - "what are you up for" and "what are you down for" can be synonyms. "What are you down for" can also be an enquiry in to what you signed up for ... isn't English great?!? Where I am we're more likely to ask "are you up for it"; though there's a slight connotation of meeting a challenge in that phrase.
Did you do any market research - perhaps ask people in the right demographic to write a tweet/text [ie SMS message] as if to invite you to an informal impromptu event and take your inspiration from there.
"Happening"? is all I got right now.
That bit you said about what Down does better - why isn't that on your landing page?
tl;dr sorry I don't really have any ideas what's better - if it resonates well with you then perhaps you're best to stick with it.
Did you do any market research - perhaps ask people in the right demographic to write a tweet/text [ie SMS message] as if to invite you to an informal impromptu event and take your inspiration from there.
"Happening"? is all I got right now.
That bit you said about what Down does better - why isn't that on your landing page?
tl;dr sorry I don't really have any ideas what's better - if it resonates well with you then perhaps you're best to stick with it.