My 9.5 month old DS has recently started crawling and pulling up on things. Since then, when I put him in his cot for a nap or for bed, he just starts crawling around his cot, or sits up and pulls himself up on the bars. He doesn't have any toys in his cot, as he's always been really good at just falling asleep within minutes of being placed in his cot. He does have a mobile, which I put on every time he goes in his cot so that he knows it's time to sleep.
What can I do to encourage my DS to sleep and not crawl around his cot?