Follow the Leader
Start by having him kneel on a soft surface, like a carpeted floor, sitting up on his heels as shown in the image.
Have her lower herself down on to him with her back facing him, so that she's straddling his bent knees. She keeps her knees bent as well, with one on either side of his legs.
Both partners can then rock together in that position, back and forth, up and down, as quickly or as slowly as they please.
This position is great for both partners because he has access to her breasts and nether region with his hands, which is exciting for him and fun for her.