"How to massage your baby." Baby is constantly building muscle and, just like us, that makes them sore. I would consider a message with some lavender essential oils to smell and candle lit. Maybe right after a warm bath and get baby relaxed for sleep.
Here is fun and interesting infographic about baby development. Physical development happens so quickly from birth to age two. It is important to know the different miles stones for physical development in an infant so you can track their progress.