I think it's a good idea to keep a photo of yourself as a child where you see it often, especially one that reminds you of a happy time. I have one of myself about 3 or 4 years old, lying in a hammock with my grandmother and both of us wearing big grins. I have a couple of cute snapshots of me as a toddler on Christmas morning that I like to put out for Christmas, along with a plate my son made that says, "Merry Christmas mom and bad"

The best thing for bringing out your inner child is spending time with a child. It's very instructive (and entertaining) to watch how they operate kind of like people but without the filter of inhibition.

I have a middle-schooler and two pre-schoolers in the house so they definitely help
keep me 'childish'.

Occassionally I like to paint or draw abstracts. I like to sing. I like to play silly games with my children....like we had this one game where we called out things that 'rocks' could do that they ordinarily can't....and really, fully picturing the image was hilarious. We would each take turns saying something new that rocks could do. For example, "Rocks take baths!"
or "Rocks drive cars!"
Or "Rocks climb trees"!

I know, so silly, and so funny! I don't know how we came to that game but they still ask to play it.