Virtually anywhere! As long as the place is public property and there are people wandering about, you can preach. Ideas would be public parks, outdoor market places, downtown city squares, fairs and festivals, waterfronts, outside of major events, on campuses, on sidewalks around schools, etc. The locations are limitless. If you have any doubts about a particular place to preach, call your local authorities and verify with them what freedom of speech rights you are allowed.
Occasionally you will find cities or regions that are generally hostile to the gospel, street preaching and even first amendment rights. In these cases, sometimes a street preacher will make it a personal mission to "break open" an area for other preachers. This means you must be willing to bring a camcorder, preach in a public place, get arrested, go to court and win. As long as you are merely preaching on public property, any city code or local ordinance will not stop you from preaching and will be struck down in court. Sometimes you must appeal until you reach a higher court before you win. It can be hard work but the result allows street preachers to invade the area without having to worry about jail time.
Even in the most hostile regions, however, campus preaching very rarely results in arrest. If campus security and or police advise you to leave, it is probably best to heed their advice.