I used to get frustrated that I couldn't get a toy to stay on his lap, however, I do use it as an opportunity for him to use his feet and legs. In fact more recently, we've come to realise that a lot of his movement is quite deliberate. So we bought him a roll out keyboard which he plays with his feet. And he drums with his feet, too. Especially in the car. The other day we were driving and we had The Who playing. Seth went crazy. All I could hear was a very excited boy and feet drumming against the passenger seat in front of him. Next time I'll pull over and video him!
If Seth is sitting on the floor and we put a toy on his lap he will wriggle and bounce until the toy is under his legs. Some lovely friends of ours bought Seth a toy guitar for Christmas. Whilst we've had fun playing with it, we hadn't had much luck getting Seth to play it. However, I was giving it to him with the expectation that he would use his hands. Last week I adjusted my thinking. Seth was sitting on the floor and after watching him wriggle a keyboard-type toy off his lap and under his thighs to play it, I put the guitar under his legs: