Use Youtube to see videos about your destination. Make a rough list of activities you would like to do while you're there. 

Use Google Flights to check travel options. Are there alternate airports you could travel from/to? For example, fly out of San Antonio instead of the Austin airport? 

Are your devices charged? Do you have your Sony Noise Cancelling Headphones for the trip? (They are a game changer)

Bose Noise Cancelling Headphones are another good option. 

Download the MyTSA App to see wait times at your airport. 

Purchase a book from Amazon's Best Seller List or download one of Amazon's Best Selling Movies for the plane.  

Search for your seat on seatguru.com to make sure it's an ok seat. Check to see if there are any upgrades available. The emergency exit row seat with more legroom may only cost a few dollars more. 

Search for your flight on FlightAware.com to find out what city your plane is currently in. You may be able to see delays ahead of time.

Check what food options are available at the airports you are traveling through.

Try AirportFoodList.com for food options at your airport. 

Check your options for getting through the TSA line quicker with TSA Precheck or Clear.

Is your TSA Precheck number attached to your boarding pass? 

Do you have your ID with you? 

Make sure your passport is up to date. Apply for one if you don't have one. 

Research if you need a visa to visit the country you intend to travel to. Apply for one if necessary. 

Pack for casual and formal events. 

Check in to your flight 24 hours before takeoff

Make sure you change money over to the local currency before you fly. Make sure you have a credit card with no foreign transaction fees. 

Search your route on Google Maps to estimate time to Airport. 

Compare Prices between Uber/Lyft, public transportation, and airport parking. 

Check the weather for the cities you are traveling through. Also check the news. Are there any potential delays? Are any airlines experiencing flight cancellations?

Look at loungebuddy.com to see what lounges the airport has.

Use Google Flights to search for alternate flights you could take to reach your destination. You want to be ready if your flight gets delayed/cancelled or if the flight crew offers big money for someone to switch flights. 

Is your layover less than one hour? It might be good to check for alternate flights in case your plane is delayed at all. 

Could you drive the rest of the way or take the train if your connecting flight gets cancelled?

Check out apps like Hitch that offer rideshare services between cities.