If you're into mountainbiking, you probably already know about Glentress - it's one of the finest venues in the country but if your just starting out, there are a number of pleasurable routes in the forest for even the youngest beginner and if you've come without your bike, bike hire is available from several locations in Peebles and in Innerleithen
Glentress is part of the 7stanes mountainbiking trails. You can find out all you need to know about mountainbiking in glentress from here.
(please let us know if this link is out of date)