You have to make sure that the play mat is safe, so your baby doesn't hurt herself when she lie on it. The blanket have to be thick enough, the arches must be well secured and you must make sure nothing can get loose and be swallowed.
You should also consider if your baby would enjoy lying on the play mat. Most babies are happy if the play mat is colorful and has some bars or arches that she can look at while she is lying on her back. Baby play mats without bars and arches should be avoided, since there will be nothing to look at for your baby unless she lies on her stomach.
It's good if the play mat has some hanging toys that your baby can touch. It's helps development her hand-eye coordination and also make it more fun to lie there. It's especially good if the toys can be detached and given to your baby to play with.
When you look at the blanket and the hanging toys, you should check if there are any built-in sounds or music. Babies love it when their toys make some kind of sounds and it also stimulates their development. You can for example find baby play mats that play several minutes of Mozart when your baby touches somewhere on her play mat.
Finally, you may want to check if the play mat fits inside your baby's playpen. You should also look at a bumper, so you baby doesn't hurt herself if you put her play mat inside a playpen.
