The key for buying a gift for someone who is a kid at heart is just thinking about toys that are hard enough for adults to enjoy them.
Products to consider
This is simply a cooler version of an actually lighter. This electric lighter uses an arc to light things. This fun and affordable lighter will definitely be a hit with any pyro.
Nerf guns are more accurate these days. A childish adult will definitely have a lot of fun tormenting people with this one.
This play sand is like more sophisticated Play-Doh. It is relaxing and fun to play with. Even for adults.
This titanium spinning top is a great gift for someone who enjoys simple toys.
These round magnets are super fun to fiddle with!
This LEGO rocket is the perfect gift for a kid-at-heart. It has enough pieces where the project is decently challenging and the completed project is a great decoration.