Vietnam Mountain Marathon, 24-25 September in Sapa, Vietnam. Now I'm not volunteering to be the organizer -- I suspect this is going to be more of a do-it-yourself operation -- but it obviously it would be a lot more fun to try to take the same Hanoi-Sapa train and to stay at the same hotel in Sapa. So here is some plane, train, and hotel info.
Air Asia: unfortunately the Air Asia flights to Hanoi from both Don Meuang and KL leave first thing in the morning (06:45 and 06:30) respectively, so if you're coming from Hatyai you'd need to spend a night in either Bangkok or KL. But the return schedule is perfect: the daily Hanoi-Don Meuang flight leaves at 09:00, arriving at 10:50, so you can catch a noon flight to Hatyai and be back by early afternoon.
Jetstar: if you want to get to Hanoi in one day, this is how you can do it. Jetstar flies from Bangkok to Hanoi daily at 14:20, arriving at 16:10, which allows plenty to time to get to the Sapa train (see below). Just one catch: they fly out of Suwannaphum, so you either have to pay the extra to fly Hatyai-Suwannaphum on Thai Smile or else take the shuttle bus from Don Meuang to Suwannaphum.
Nok and Qatar also fly to Hanoi from Don Meuang and Suwannaphum respectively but their times are about the same as Air Asia and Jetstar, so you'd still have to spend a night in either city. If you'd rather spend the night in Singapore you can fly Tigerair from Hatyai to Singapore daily at 14:30 and then catch their flight to Hanoi the next day at 13:00.
The two express trains from Hanoi to Lao Cai (from where you can catch a cab to Sapa) leave at 21:40 and 22:00, arriving at 05:30 and 06:10 respectively. Return times are 20:15-04:20 and 21:05-05:09. There are also lots of buses throughout the day, some quite fancy and comfortable, so that might be a better way to do the return journey.
For booking train and bus tickets in Vietnam I can highly recommend Vietnam Impressive. Very efficient and helpful service.
I've never been to there so can't vouch for it, but Sapa Dragon Hotel looks good, and it's just down the street from the start line. It's also big enough that they'd probably have rooms for all us. (Many hotels in Sapa seem to have only five or six rooms.) If nothing else we should coordinate on staying at the same place. And this one I will organize, if you want.
Thais don't need visas to Vietnam but us honkies do. The last few trips I've used myvietnamvisa.com, which despite the silly name does a great job.
It's still three months off but we should start talking dates pretty soon.