Just a funny story for you: my daughter's bedroom is on our first floor, so it also serves as a playroom. There are a ton of toys in there, of course all accessible to her. Before I put her down for her first sleep in her big girl bed, I made sure everything was put away so the toys wouldn't look so inviting. Well, I shut the door and heard no sounds, so I thought everything was going great. I peeked in about an hour later to check on her, and there she was, in the middle of the floor, surrounded by all her toys and about half her clothes!! It was hyterical, but I new if I laughed that I'd have this happening again, so I held it together. Instead, I told her in a very sterm voice that she was NOT allowed to get out of her bed without me in the room and that if she needed something she had to call for me. I think my voice came out more stern than I intended it to because I was also trying to hold in the laughter. But I think this turned out to be a good thing because to this day (and she turns 4 soon), the child still wil not get out of her bed unless I tell her it's ok!!
On small piece of advice - I would choose bedtime over naptime because that's when my daughter gets to sleep the quickest. She takes longer to fall asleep at naptime. I would suggest that you choose whichever time you find your child falls asleep more quickly and easily.
Good luck to you! Before you know it, she'll be in the big girl bed without a problem.