My happy place is definitely the sea so I've gone for a theme of the seaside with inspirational quotes written on pebbles amongst pretty shells. It's completely up to you what you want your jar to be full of though, you might choose dried flowers or past tickets from events etc.
I was inspired by the idea of writing quotes on pebbles after finding a pebble in a little trinket shop with the word "zen" written over it. I bought a couple there and then as I loved the cute messages on them, but have since been to my local beach to customise my own. You could even carry a certain pebble around with you as a lucky charm with the message you are needing to get you through that particular day.
"If light is in your heart, you will find your way home." - Rumi
Lots of love,