How to keep toddler in toddler bed (transitioning from crib)?!

We are expecting our 2nd baby, our first is 19 months. He sleeps so well at both bedtime and nap time (goes to bed between 8 & 8:30, doesn’t cry unless it’s for about 30 seconds, falls right to sleep or is at least doesn’t cry, wakes up around 8 and is happy as can be) so I almost don’t want to mess with it! I know he is still young. That being said, we have converted his crib to a toddler bed (we also have a pack & play that he can’t escape from as back-up). Rather than buy a new crib, we are planning on converting the toddler bed back when the newborn baby outgrows the bassinet, so we are trying to transition our toddler before the baby arrives so that he gets used to it.

My question is…how on earth do you get them to stay in bed?! Or how do you handle it if they don’t? So far, he cried, got out of bed, knocked on the door to be let out, then sat down quietly and read his books. If I were a kid, I’m sure I would do the same with that freedom! I don’t want to just keep putting him back in the pack & play because I feel like he will never transition if we do that. We are only on day 2 of trying so I’m not worried at all, just trying to prepare to be persistent but am wondering how to do so!