Make them better coffee and then don't comment on it until they ask.
Most people seem to connect bitter and strong and it's a hard habit to break out of. People are used to crappy coffee.
Just getting them to a stage where they buy quality beans and grind them fresh is a big leap, but once they reach that baseline you can introduce roast levels.
Oolimo, the website and phone app is a great resource for me. It lets you enter notes on a fretboard to identify chords.