As someone who’s the owner of a national conference business my tip is branded chargers. Literally everyone needs them and is incentivized to keep it around.
Really anything that is handy for business travelers is going to land well with a lot of people at conferences.
I used to take these but no longer do, since I don’t want to risk any shenanigans anywhere in the supply chain.
Otherwise, stress balls and bouncy balls are always a hit to bring home and give to the kids.