I have yet to work for a company that buys me lunch, much less annual ski trips. Hell, coffee hasn't been provided at any of my jobs, I've always had to pitch into a coffee pool just to have it. That's not to even mention getting to go to conferences. If I wanted to attend a conference, it would be coming out of my wallet and vacation time.
If you want me to collaborate, put it in on my schedule and pay me to do it. I'd say that most of these "spontaneous collision of people and ideas" happen during lunch. Guess what, that lunch which I am already paying for, is also unpaid time. Ever since I've been working from home, my lunch hour is mine. I get to unwind, read or watch tv, all while eating a healthy meal.
Finally let's talk about things that would get me back in the office. Shorter days, I have to commute a total of 2 hours a day, so I want my day to be 2-3 hours shorter. An actual office, with a window, door, temperature control, and some kind of noise isolation. I don't like working in those open office bullpens where I get to listen to salesmen screaming on the phone or secretaries sharing the latest gossip. Start screening people for personal hygiene. I get so tired of having to work with a heavy smoker or someone who can't be bothered to shower before showing up to work.