There are a few things in the games that seem reminiscient of Hit the Road, Day of the Tentacle, Maniac Mansion, Monkey Island, etc. if you saw or heard anything in the new Sam and Max games that reminded you of these classics, list them here!
Obviously, don't mention deliberate references to the old games ("I can only hold my breath for 9 minutes!" "Did you just get deja vu?" etc)
For me, the "Messing with the Soda Poppers" theme reminds me of Laverne's theme in Day of the Tentacle (which plays when you select her, put the White-Out stuff on the fence, tempt the cat with the toy mouse etc). The music that plays in Stinky's 1980 is reminiscient of Maniac Mansion (though that's probably just because of the extreme 80's-ness present in the game) and the River Styx music reminds me of the tunnel theme from Monkey Island 2's last chapter. (though everyone mentions that)