A pendulum, left alone, will oscillate for very long time. I once observed a hanging paddlelock, unintentionally hit, oscillate for a couple of hours. There are even special conditions of lubrification and vaccum that would make it do so for even longer. Friction losses are really small there.

The idea is to make a pendulum like a pirate ship, an attraction on amusement parks. It doesn't have to be that big though. On the bow tip, you attach a long permanent magnet. This permament magnet should always travel inside an external coil, as the pendulum makes it way.

When you want, you can energize the coil and repulse the magnet. The magnet and the coil field will interact. This is storing energy.  When you want to retrieve the energy, you attach a load into the wires of the coil.

So, is this energy storage system any good? Does it have, perhaps, good energy per kilogram ratio? 

[UPDATE: I'm suspecting that the way magnets interact could ruin this idea... perhaps a field coil, maybe? needs more thinking]