There is some xc riding up the Glennifer Braes - possible to ride from the back of Elderslie to Barrhead covering sides of Robertson Car Park past Paisley golf course Glen Park, Brownside Farm and Firenze . This is mostly all traditional xc mountain biking rather than specific trails so can get muddy when its wet. There is some hardpack too which stands up to the weather better on the Brandy Burn side towards Elderslie at Windyhill Woods
http://www.youtube.com/watch?v=x_svn4JmfZU
At Clyde Muirsheil you can follow some of the paths and go up to the old mine, at Lochwinnoch there is Parkhill Wood there is a short loop in there.
Fairly close by there is the Commonwealth Games course at Cathkin Braes (which is closed just now for the games) and miles upon miles of access roads at Whitelee Windfarm plus a small mountain bike skills area which is due to open soon.
Further afield then Glentress down in Peebles is the big one. Purpose built trails to suit all levels of ability, would definitely recommend a visit there.