Airport

The airport is unlocked at city level 14. It allows you to trade one resource(Materials, Energy, Food, Iron and Technology) for another, effectively allowing you to buy resource upgrades five times faster. It also allows you to trade your technology for one of the other four main resources (that can then be sold at port if desired).

The airport and food upgrades
Since the airport trade rates are based off of your current production rates of each resource, food upgrades can have a dramatic effect on the value of your saved resources. Understanding this is key to efficient use of the airport. Buying a 5x production upgrade when you have 5 hours of that resource saved effectively reduces the value of that resource to one hour. By first trading the resource for any other resource you can avoid this, and have 5 hours of that resource at your new production rate (by trading back after you've bought the upgrade).

This technique works for all resources except food, since you can't trade away food before spending it (and you can't trade a fraction of a resource- it's all or nothing). Try not to have more food than you need to buy the next food production upgrade, as any remaining food will become much less valuable.

Other tips for the airport
Generally you want to rush food upgrades first. If you're anticipating another city level before your next reset, you may want to focus on whatever will bring in money faster instead, as all of your resource production will automatically increase when you get the new city. This could even include selling your resources, if you are closer to the next city level than you are to being able to afford another resource upgrade.

After buying all the food upgrades worth having, port levels (iron) and profit paradox research (technology) are typically the best value for your resource. Don't bother wasting a lot of materials on a service sector business that's not producing much and not near another trophy. Likewise, super expensive energy upgrades should be reserved for only the highest earners.

Each resource can be sold for 12.5% of your current income at the port, meaning that with the airport unlocked, all resources can bring in an additional 62.5% of your income. This also means that spending, say, 10 hours of resources is like losing ~6 hours of income. So it would only be worthwhile if you plan on idling for long enough to make up the difference or if you already had more resources saved up and the boost to income increased their value by more than the cost. Spending all of your resources on an upgrade (or several useless ones) right before a reset is a bad idea.

Also, don't forget to trade your technology for another resource (doesn't matter which one) right before resetting so that it will get automatically sold at the port. Note: Using an "Instant claim fee" does not sell your resources, so you do not need to do this if using one.