Tbh it's all about what helps you to relax. My answer could be completely different from yours.
For example, I really love listening to soft music with the sound of rain in the background. Pull up some videos on youtube from Brad McBride or go to rainymood.com and just kick your feet up with some hot tea. I even turn the fan on to simulate the chilly breeze that comes with rain around my area and wrap myself up in a big blanket.
I also really like going on LOOOONG walks, often for hours at a time. I listen to music for most of the time until my ears start to hurt, then I pop out the ear buds and have sort of a zen experience just listening to the things going around me and continuing with my walk. There's also a few nature trails I go to around my house.
In terms of a relaxing little day, get padded up and do some coloring or drawing. They sell adult coloring books at most stores now. Cuddle up with some plushies and watch a movie that makes you feel little. I just discovered that movie 'Inside Out' a few days ago and it instantly became a favorite. That one will stick with me for a long time. Make your favorite little food, or roast some marshmallows over a stove top (If you have a gas burner with an open flame). Play with some little toys like cars, or dolls, or building blocks or legos. Or just snuggle up for a nap with a nice book. You can even find a few popular children's books being read on youtube. Have a bedtime story read to you and doze off with a paci in mouth. Maybe keep a warm bottle of milk next to you as well. Maybe get padded and go to the park and swing on the swing sets. Or go somewhere new and exciting like an amusement park. Or just sit and home on a few nostalgic video games while padded. Or bake something! I'm not a big baking fan, but there's something special about whipping up something yummy and smelling it as it cooks and spreads its deliciousness throughout the house!
The possibilities are endless. The question is, what works best for you? These ideas are only ideas that I would like, but it's up to you to decide what you think will make you feel extra special or little.