I used to have extended international business trips where I'd be solo over the weekends in large foreign cities. I loved having my own free time to explore and do whatever I wanted on my own schedule.
I think a lot of common sense advice applies - things like be alert and aware of your surroundings, program emergency phone numbers into your phone, guard valuables (or don't take them with you), buy and use a pickpocket proof bag/wallet, use maps discreetly when out and about (plan out routes, destinations in your hotel room and duck into stores if you need to look at a map on your phone), don't go anywhere with someone you don't know or invite them to your hotel, etc.
Text someone you know - whether in country or even at home - your daily plans and destinations and check in with them via text throughout the day. If you don't speak the local language, ask your hotel concierge to write your destinations or directions in the local language for you to use for taxis or public transport if in a location where it's generally safe.
I also scan or take pics of my passport information page, credit cards with the # and out of country customer service phone # and either email them to myself or upload to a secure online storage site like Dropbox that I can access from my phone or a computer if needed.
When I first started traveling solo like this I'd also look up the location and phone # of the U.S. Embassy, just in case.
Have fun!