I've used denatured alcohol (aka methylated spirits) for over 30 years with good success. It's safe for most plastics, with ABS (changes the color) and Lexan (makes it cloudy) being the exceptions I know of. It's readily available, dries quickly and is relatively cheap. I use it with a small, stiff plastic-bristled brush to scrub the boards after soldering.
Isopropyl alcohol also works well. Make sure you get actual high-concentration (85% or better) Isopropyl (or Isopropanol) and not the commonly available "rubbing alcohol", which contains glycerine and will leave a residue.
DON'T be lazy and just soak the boards in alcohol. Despite being relatively mild solvents, given enough time they will disolve a lot of things.
There are also many commercial electronic spray cleaners that remove flux. Some will severely damage (even dissolve) plastic despite being labeled "safe for most plastics". They never seem to say exactly what plastics are included in "most", so I recommend you test them first.
The older formulations from 20 years ago worked great but were either carcinogenic or ozone-depleting and have been banned. The newer concoctions don't seem to work as well to me so I've gone back to using alcohol and a brush.
BTW, use in a well-ventilated area and away from spark or flame. Alcohol flames are nearly invisible.
Isopropyl alcohol in a spray can. Probably cheaper to just buy a bottle.