The following websites have been valuable for me in my travels.  The pandemic, however, has changed the world of travel with many businesses cutting back or shutting down.  Please let me know if you find a link is not working.
AIRLINES
Buy airline tickets at least 7 weeks in advance; Tuesdays or Wednesdays are the best day to fly; avoid Fridays.
United Airlines     Air Asia     Easy Jet     Google Flights     Kayak    
Priceline     Costco
    Low Cost Airlines
HOTELS Check customer ratings on Tripadvisor then check rates on the hotel websites.
Hotels.com    Booking.com    Expedia    AirBnb    Trivago    Orbitz
CITY PASSES Passes save you money and get you into express lines.
Paris Pass    Athens Pass    Singapore Pass    Trip Guru   
Trip Advisor City Passes

PACKAGE TOURS Package tours handle all arrangements and can save you money and worry.
Gate 1 Travel    Travelzoo
CRUISES
Cruises start discounting 90 days before departure and booking a few weeks before can save you 85% or more!
Vacationstogo        Cruise Critic
TRIP INSURANCE Trip insurance is recommended.  Folks over 75 are often not accepted for coverage.  Some premium airline cards offer some limited coverage.
Allianz    World Nomads    Squaremouth
TRAVEL INFO These sites and the State Department can help you choose where to go or to avoid.
Condé Nast Traveler    Afar    Tripadvisor    U.S. State Dept. Advisory
RAIL TRAVEL Don't overlook rail travel to really see the countryside.
Seat 61
RIDE SERVICES Be aware that some cities and neighborhoods do not allow these services.
Uber    Lyft    Grab
TRAVEL APPS
These phone apps are invaluable.  I recommend in getting a SIM card and time for your cell phone if you are staying in a country very long.
Skype    Whatsapp    Hipmunk    Skyscanner    Accuweather
Moveit Maps    TripIt Itinerary     Google Translate
COVID Check the State Department Update on countries closed to tourists.
Country Specific Information
I have posted videos of several of my travels on YouTube.  You can view them by clicking the blue links:
Peru         My Oaxaca     Mexico City     Baltic Cruise     Colombia