Enroll in STEP and check travel advisories
The STEP Program (Smart Traveler Enrollment Program) at the US State Department STEP allows you to sign up and register your travel destinations and lodging addresses. You will then receive emailed notices for your destination, including notice of planned demonstrations, security and safety alerts, health information and weather alerts.
The STEP program also helps the State Department and local US Embassy know where to find you in case of emergency. You can also check this website for travel advisories for the country or countries you are visiting. I always check this but don't let it necessarily deter me. The only time I have had my wallet stolen was at my local Trader Joe's, so security concerns exist everywhere.
Check on ride share availability at your destination ahead of departure.
A quick google search can advise on ride share availability as well as how to avoid being scammed by unscrupulous unofficial taxis. Besides Uber and Lyft, download the Freenow app for black cabs in London. Download the Bolt app for coverage in 850+ cities worldwide.
Check into international calling with your cellphone provider.
Your carrier may provide access as needed, but you may have to sign up in advance. AT&T charges $12 per 24 hours when you access roaming. This is useful if you need to summon an Uber or check a map and you aren't near wifi. It wouldn't hurt to know the equivalent of 911 for each country you are visiting.
Safety considerations
As a solo traveler, there is only one of you to keep watch of your surroundings. Sometimes I actually feel safer by myself, as I am not distracted by conversing with a travel partner.
It's always a good idea to review your plans for the day with your hotel staff and ask if they have any suggestions or cautions for you. As a solo senior, I usually don't walk around late at night. Even if I walked there, I'll often take a cab back from restaurants in the evening.
Carry copies
Upon arrival at my destination, my passport, valuables, and my additional credit/ATM card go into the hotel safe. I bring a total of two credit cards with the one in the safe as a back-up, so my trip is not ruined in case of loss or theft of my main card.
I have a laminated copy of my passport picture page that I carry walking around. No one has ever required more than this for ID, and if I was knocked unconscious in a car accident I could be identified.
And, ever since I was initially taken to the wrong hotel in Edinburgh, I also print out multiple credit card-size pieces of paper with my hotel name, address, and phone number to stick in my wallet. These have been much-appreciated by cab drivers who may speak neither English nor the local language.
Safeguard valuables
The most important things you are walking around with are your credit card and cellphone. Make sure your cellphone is programmed to backup to the cloud when you have access to wifi. Don't set your cellphone down anywhere or leave it out on a table while you're eating.
I used to carry a wallet full of credit cards and stuff I didn't need with me. With the advent of card holders, I now carry only one card, my medical info card, and my hotel address in my tiny card wallet.
Buy travel insurance
This is a must. It usually needs to be purchased around the time of making your first bookings for the trip to ensure coverage for preexisting conditions. US-based health insurance, including Medicare, does not cover you outside the US. My credit card company offers trip insurance that includes trip cancellation, lost luggage, etc, but does not provide enough accident, health, and evacuation insurance for my comfort. If you travel a lot, like I do, it may be cost-effective to purchase an annual plan that covers all trips for the year. A good place to search for and compare plans is https://www.insuremytrip.com/.
Check the CDC website for travel and vaccine recommendations
The CDC posts health and vaccine recommendations for countries around the world. Consider whether you want to see your primary care physician or a travel specialist for preventive care and prophylactic medications/vaccinations, especially if going to far-flung locales. My local county health department has a travel clinic that provides health recommendations and offers vaccinations for a reasonable cost.
Make a “medical info card”
I recommend carrying a laminated card with your name, birthdate, cellphone number, medical diagnoses, medications (with dosages), and allergies. Medical providers the world over can read prescription medication names and often understand written diagnoses even if they don't speak English. I have had health emergencies in Mallorca and Stockholm which were easier to negotiate with this information. And as a postscript, I received excellent care in both places and travel insurance covered me both times.