Renting bikes in Hong Kong
I've tried renting bikes for riding around in Hong Kong - including on Lantau and Cheung Chau, and at Tai Mei Tuk in the New Territories - and I've been well impressed with the quality of the bikes, and the rental charges, which so far in the range HK$20-30 for a day per bike (maybe a little extra if also rent a lock; you should probably bring your own helmet).
There aren't many places you can do this (see comments for fuller list); those I know of are:
Cheung Chau (including a place a little left of the ferry pier as you arrive). This is hardly the best place for cycling - as there's a relative dearth of easily cycled roads, the most popular being alongside the harbour to the southeast of the island, as well as north along the harbour and then a beach to the northeast coast. But if you just want a relaxing ride, or you're game for a few low hills and perhaps even carrying the bike up or down some flights of stairs, it's a fun place.
Better still, I think: hire a bike on Cheung Chau, and take it on the ferry to Chi Ma Wan on Lantau. From here, you can readily ride (and walk, if you're not good uphill!) over to Pui O, maybe following paths through the water buffalo fields (the buffalo look imposing, but are well used to people), before joining the main road along Lantau's southern coast.
Mui Wo (Silvermine Bay) on Lantau also has bikes for hire, including from Friendly Bicycle Shop, near ParknShop (on left, just beyond the bust station, as you arrive on the ferry). Contact: 2984 2278; as well as the $30 a day ordinary bikes, they also have good mountain bikes at $150 per day.
You can potter around the paths and small roads along and behind Silvermine Beach: there are small houses set amongst fields, and if you head to the northwest of the Mui Wo basin, you should find the Silvermine Waterfall. There's a steep path up beside this; beyond a more level concrete path wends along a valley, into the hills, and is interesting for exploring (as far as the next steep uphill stretch, anyway!)
To head west from Mui Wo, you'll have to go up and over a hill, then through Pui O, following the road that switchbacks along Lantau's southern coast. The scenery is rural; a highlight is the stretch above Cheung Sha Beach, where it's perhaps tempting to stop for food at a beachside restaurant such as The Stoep.
Tai Mei Tuk in the northeast New Territories boasts several cycle-hire places; it seems competition is fierce (I was told prices are down from HK$100/day a year or two ago; to HK$20/day, or even HK$5/day for kids' bikes). (Many people hire bikes in Tai Po Market, then ride to Tai Mei Tuk; I think it may be best to ride a bus to Tai Mei Tuk, saving your energy for cycling through the best scenery.)
You could likely cycle southeast, along the dam of Plover Cove Reservoir - should be a pretty easy route. But, it's perhaps most interesting to head northeast.
There's a low hill, then a smooth stretch alongside the Plover Cove Reservoir. When I tried this route, I stopped off at Chung Mei, where the main stream enters the reservoir; then cycled uphill till another stop near Bride's Pool Waterfall, and continuing to Wu Kau Tang, for a stream side stroll to the top of Mirror Pool Waterfall. That day, the sky was simply glorious; and what a sunset!
If you're interested in mountain biking: there are rather few trails where this is permitted in country parks, and you need a permit - for info, see Mountain Biking Activities; also the Hong Kong Mountain Bike Association.