One of my favorite stores for basics is HM. Their basic collection is low priced and reasonably good enough to last you through the season. It's composed of short sleeve tees, long sleeve tees, onesies, hoodies and sweats. They are available in packs or they usually have buy one get one free in both baby and kids sizes. This winter I plan on picking up some of these basics for Leyla for a casual day.
Here are a few shots of Leyla at the park today wearing a basic pink onesie from HM. Their sizing usually runs big so she is able to fit into 18/24 at HM. What is great is that you can take a basic onesie and make it into a cute outfit. A pair of colorful Zara shorts dressed up Leyla's plain top perfectly! As you gradually progress towards the end, you will see Leyla was a bit upset because she didn't like getting wet on the slide. Aww.