I vote otter.
They will do their slide even across a flat surface from what I have read.
I have seen and photographed otter slides on grassed slopes adjoining bogs and it is an amazing phenomenon.
Otters are animals that appear to do things to purely to have fun, which is out of the normal range of activity for a lot of aninmals. Most wildlife behavior is related to survival, courtship and reproduction.
I have read a lot about the otter and they are apparently doing very well in most of their range.