You think you're taking it easy and you don't plan too many activities in one day. And yet your baby is overtired. How is that possible?
Think about the following.
Your baby has been in mom's belly for 9 months. That was a protected cocoon in which it was dark and the sound was muffled.
Then your baby is born and suddenly everything is different. The light is bright, the sounds are loud. That's a lot of stimulus for your little one. And on top of that, your baby can't filter these stimuli yet. He doesn't know yet what is and isn't important. So it's not surprising that this quickly becomes too much.
Your baby gets overstimulated and overtired. A baby who is overtired sleeps less, is therefore not rested, then has little energy to drink, does not feel well, cries more and then has difficulty falling asleep again. In short: you are in a downward spiral.