My GSP comes mountain bike riding with me. We have a place only 10 minutes drive from my house where we can ride fire trails and single track all day. It's the best way to maximise energy expenditure in a short period of time without expelling all the owners energy in the process!!
We cover a good 10-15 klms each time and she is always exhausted that night. So, If I need to tire her out I can sneak out for an early ride and she then tends to sleep most of the day rather than be bored or destructive in the back yard.
I will have to wait another 5 or 6 months before I can do it with my new V. he's only 6 1/2 months old now and it would tax his joints too much at this stage.