I made a post in the Hints and Tips topic regarding this, but here is a short version of which adapters I have found to work undocked:
IHome USB-C to USB-A adapter (this is the one I'm currently using)
AmazonBasics USB Type-C to USB 3.1 Gen1 Female Adapter Cable
Nintendo Switch Lite Dual USB Playstand By HORI - Officially Licensed by Nintendo (also works for the regular Switch)
HORI Nintendo Switch Multiport USB Playstand
These are the two keyboards I use with my switch that work fully:
Logitec K360 (wireless with small USB-A dongle)
Magiforce 68 Key mechanical keyboard (wired, USB-A)
In general I would avoid keyboards with lots of extra functionality or anything that might draw too much power (anything that might require more than a standard HID driver or that has a lot of lights or USB ports that might drain your battery or require more power than the switch will provide).