(sunny locations) I always love the idea of using succulents in unique containers. For it to be hardy in Ontario, I would use sedums and hens and chicks. If you are daring, in summer use various echeverias which are not hardy and need to be taken in for winter.

or if it has a deep hole in the middle of the trunk, I'd plant a dwarf form of clematis.

Or if you feel cottage garden like -- strawberries draping off the sides would look great and taste good too.

Shade location. Use creepers and golden creeping jenny in large numbers so that it looks like a golden waterfall. Add small upright plants to add interest. Plant a small tree in the middle. Add sheets of moss too. - what else.... just about any small plants that can grow in woodlands like ferns as well.