The arena probably to do with their hands. If you are in the room there is no space, you can use a cot. It should be noted that for the first steps child needed a solid Foundation, which will serve as a hard mattress or a plywood bottom. Bumpers should be raised high enough that the baby could not get them to roll over. The disadvantages of such use of the crib is its small size and the lack of separation of sleeping space and games.
At small dimensions of the room you can also consider the option of precast arena, i.e. to use some angle to temporarily transform it into a safe enclosure. The two sides are closed with walls, one - some furniture - a cot, a dresser, and the latter temporarily separated by a sheet of plywood or plastic. When the arena is not used as intended, this part is simply removed and does not disturb anyone. In this case, it is necessary to make sure that in the resulting structure there is nothing dangerous for the baby, and all structures are securely fastened to withstand, if necessary, the pressure of the baby.
If the room dimensions allow, you can make a spacious arena, which will either be stationary, or move around using wheels - in this case, you must have a solid Foundation. The design can remind the crib and consist of upper, lower and vertical slats. It is necessary to take into account the height of the fence is to make it at least a meter to avoid loss of there child, the distance between the rails should be small, the baby is not stuck there. The dimensions and shape of the arena can be absolutely any, the main thing, stable. The presence of doors in this arena will make it more functional and convenient to use.
You can do arena with solid walls, it is necessary to use only safe and non-toxic material - wood, quality plastic, etc. Iron sheets are very hard for a child and unable to hurt him - in this case they should bind/to wrap something soft: foam, wool, etc. As an option, walls to stretch small mesh, tulle or gauze, as long as this fabric does not SAG, and the falling of the baby, a little cpuinit.