• 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 data is wrong. Then I put on osm the corrected coordinates and appropriate tags. Some of the maps i use are:

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