If you visit a Montessori Infant classroom, you will find that there are no "containers." That is, there are no cribs, playpens, pack n plays, bouncers, walkers, or anything that you would put a child IN. This is because Maria Montessori studied the value of freedom of movement in the development of the young child. Even the youngest infant moves in some way -- beginning with stretches, kicking her feet, turning her head from side to side. Making sure that your child is not restricted in her movements allows her to develop to her highest potential physically. In the classroom, you will find floor beds, soft mats on the floors, soft areas to climb and explore. At home, you can provide the same experience, but you need to do it in a safe way.
Both of my children have slept on a mattress on the floor from the time they came home from the hospital. You can use a crib mattress if you have one, or go straight to twin. We use twin mattresses, because they provide the extra space for movement during sleep. Make sure you have a soft surface on the floor next to the bed, because your child may roll off when she's older. L used to roll all the way across the floor while holding on to her lovey, and we'd find her still asleep on the other side of the room the next morning! Having the mattress on the floor means your child can climb into the bed by herself once she is crawling, allowing her to develop independence in her freedom of movement. If you are using a floor bed, you need to make sure that the entire room is safe for your baby because she can move around in it unrestricted. Attach all furniture to the wall, cover all outlets, and make sure there are no wires for her to get tangled up in.
My children have never been in a playpen or pack n play (except to sleep in on vacation). This means they have had free range of the entire first floor, so I had to make sure it was completely safe. N is still very young and not yet crawling, so I simply lay out a blanket on the floor in the living room for her to play on. Once L started moving, I put fragile things on high shelves and made sure there were plenty of things available that she was allowed to touch. I put a gate up so she couldn't go up the stairs, but that was it.
I do have a bouncer for N, because at home your sole job is not taking care of infants -- there are many things to get done in the day, and you can't always be holding a baby or leaving her unattended. However, she is never in the bouncer for very long. Once N starts rolling around more and crawling, the bouncer will go back to the attic.
Walkers really don't do much for your child developmentally. There have been studies that they may actually impede your child's physical development. Instead, get a push walker, like this one. These provide the support needed for early walking while allowing your child to gain confidence in her new movment.