Under or overstimulated newborn? When/how do you play?

How do you know which it is, and what are you doing (or not doing) with your newborns? My girl is nearly 5 weeks and due to issues with reflux/gas she frequently tires herself out after a feed, which can take 45 minutes or so with breaks for burping and crying. At the same time, I worry that I’m not stimulating her enough since all she does is chill in her nursery or in our living room looking at me! I’ve tried giving her some black and white cards to look at, and I read her stories sometimes or talk, but we don’t ‘play’ as such. Others - how are you engaging your baby and at what age did you start to play with them? How do you know when they’re under stimulated?