• Adding traffic lights, stop signs, 1-way streets, turn restrictions, max speed.
• Straightening streets and intersections.
• Removing arbitrary orthography in unofficial languages. Keeping only name=, int_name=.

I use external maps exclusively to be able to go there, as every other user. Too often the existing data on other maps is wrong. I verify the data and put on OSM the corrected coordinates and appropriate tags. Some of the maps I have used are:

• chargers for electric vehicles https://chargemap.com/map https://plugshare.com/
• vegetarian and vegan restaurants http://happycow.net/
• organic street markets http://bioagores.org/
• places accepting crypto currencies http://coinmap.org/
• recycling containers http://antapodotiki.gr/ https://www.ab.gr/storelocator#query%3D%26store%3D%26facets%3DstoreService%5BRECYCLE7ITEMS%5D