Garden ramp access ideas

[limax2]

I wouldn't use anything wooden gets very slippy when wet.

Very true! However Gringo's suggestion of a couple of planks seems an easy way round it if you could fix something onto the planks to provide grip. Some of the wooden bridges round here for cycles and pedestrians have wire mesh attached for grip, but probably not be good enough for a motor bike on a steep slope. Someone may be able to suggest something better?
I think you would also need to provide some sort of retainer to ensure the planks don't move when in use. Final thought, two scaffolding planks together will be very heavy so maybe a couple of removable retainers for that as well.