My daughter is a bit older but Ive had her on her back on a playmat or sheepskin from a few weeks old. I use a bouncer occasionally but developmentally - flat on the floor with toys (or nothing!!)around them is the best thing - the can explore, kick legs and gradually will end up rolling etc. I realised a few weeks ago that I needed to give her more time on the floor and she loves it! Tummy time - a few minutes a time building up to longer periods is important too!