Hi, J. --
We were lucky, I think. Our daughter only crawled out of her crib one time - when she was about 2 - and never did it again, so her crib was nice, safe confinement for her and we got to choose the time to transition her to a toddler bed.
My husband and I decided to tie the transition to her completing potty training... as a reward, basically. We told her that, once she was using the potty like a big girl, then she could have a big girl bed because she'd need to get in and out to take herself to the potty. She completed her potty training when she was about 2.5yrs, and she was VERY excited to get her big girl bed.
She phased out her naps around the same time, so there wasn't really any reason we needed to make sure she stayed in bed in the afternoons. We also had a pretty much set-in-stone bedtime routine, so we've had no trouble with her getting in and out. Maybe that's something to really solidify before the transition so they don't really think about deviating from it to get out and play, etc.
Personally, I think it's best to move to the toddler bed when there's a real reason to. If the child can understand that it's a privelege and it's got a responsibility attached (such as going to the potty on their own), I think it's more meaningful... really signifying that they're a big kid, and that's something special.
My daughter's bed is against the wall, and only one side is open to get in and out of. She's never even been close to falling out, but they do sell different kinds of guard rails that you could try. We bought some but returned them because we never needed them, even when we traveled.
Oh, also - I'd say, if your child is insisting on getting up and playing, one solution could be that they lose some other privelege that they'd otherwise enjoy. For our daughter, if she misbehaves, she loses getting to have a book read to her before bed. She LOVES books, so that's a major deal to her to lose that special thing for the night.
Good luck!
H.