Hm, I have several suggestions!
First off the classic hair brush, the cheap dollar store kind with the large plastic beads on the ends of the bristles, great on their own, devastating with lotion/oil.
There are these plastic scalp massager type hand brushes, soft plastic tines, much larger.
Also check out the sports style massagers, often they have a sort of multi head brush style attachment that vibrates, as a recipient of that toy, I died hahahah
Feathers are a great option for those sensitive, and the quill end gives a nice pokey bit
There are these gardening gloves that have plastic claws and can be great, especially in lieu of fingernails
I've seen people have success with those shed mitts too.
Broad tip Felt tip markers are killer, and make for fun Ler games "sit still while I draw this or I have to start over" haha
As a nice follow up to the markers, a nice hot soapy scrub with a bristly brush to clean off all the marker is nice
Getting into spicy stuff, google a wartenburg wheel. For the right Lee, those things are absolutely anathema to not laughing hysterically
Vibrators with the right tip can also tickle like mad.
Don't forget the power of sensory deprivation and mind games to enhance sensitivity