What do you want to know about Lantau? Since I've circumnavigated it in a kayak, I can tell you about its surrounding water
Day trip should be cool. There is a private beach along the NE shore (藍鯨灣... not sure what it's officially called in English, but translates into "blue whale bay") with a row of lange chairs, a half-developed "park" with ponds and boats and lots of trees, and quite a few row boats parked around it. However, it's only accessible by hiking or by boats (and I heard it was somewhat damaged in the 2008 typhoons... I wouldn't know, I visited in early 2007.) If you're trying to avoid the public (for rowdy party or skinny dipping purpose, for example...), this is probably the place to go.
Lots of people go fishing under the bridge between Lantau and the airport, so if you like near-shore fishing, you should have no trouble finding friends. And it's only a few minutes from the Tung Chun MTR station.
Cheung Sha is pretty nice too. But "Fun Lau" (分流) is no fun if you are on the water and the wind is coming from the west or SW. One of the choppiest sea I've ever seen in HK -- easily beats the waves around Waglan, Lamma south, or Ko Lau Wan.
Been to Mui Wo by bus ... ugh, the bus ride was no fun.
Can't recommand camping though, but if you must, bring ear-plugs. Flights are 24/7 and you can hear them pretty much any where along the north side.
What do you have in mind? Since I live on Disco Bay I might join ... but not particularly interested in bar-hoping, since I drink like a girl