You can do gigs (minor side jobs) and reported crimes to grind xp. Gigs are yellow exclamation points on your map (though so are main missions, and vehicles you can buy. Hover over them with your mouse to make sure it's not a vehicle purchase). Crimes are the light blue symbols, there's a few different types. Most of them are short and quick (4-8 enemies), the blue skull ones are organized crime locations which are a little bigger (10-20 enemies) and can have some cool loot lying around.
If you don't like those you can find spots where there's gangoons standing around and just run through the neighbourhood killing them repeatedly. There's plenty of YT videos with good locations and routes.