The Tekapo Circuit combines the Cowans Hill Track and the Lake Tekapo Regional Park Tracks.
From the visitors centre ride down over the bridge and immediately turn right up Cowans Track. After 100 metres or so it appears to turn left - it doesn't - go straight ahead up the river bank. You'll soon reach the top of the hill after a short steep pinch. Then it's relatively flat singletrack to the highway. Follow the track down beside the highway keeping an eye out for the place to cross over to the pine forest which is the Regional Park.
There are several kilometres of singletrack and old forestry tracks in the park and a few map boards but they don't have all the trails on. Don't worry, it's a small area and you won't be lost for long. None of the trails has any eloquent technicality. You won't find rollers, berms, jumps or other fun stuff but it's pleasant enough riding for an hour or so.
When you've had enough cross the road to the shore of the lake and head back to town. Be sure to join the throngs of tourists and take photos of the famous dog statue and chapel.