Verified answer

You're not likely to improve fuel efficiency much by adding things to a car, especially if that car's made by Honda (yours is). I would suggest a "performance" air filter or after-market air intake. If you decide to go with the filter, you can remove the air intake plumbing in front of the air filter box that is only designed to keep the car quiet. This can make the car quite loud, depending on how it's shaped. You'll normally get the most bang for your buck from an aftermarket intake.

An aftermarket computer is unlikely to do much for MPG on a modern Honda, especially if almost everything else is stock. Headers and exhaust can improve fuel mileage somewhat, but are rather costly and probably wouldn't save you enough money on fuel to offset their expense.

The best thing you can do for fuel mileage is to subtract weight (mass) from your car, especially rotating mass. Unload any unnecessary stuff from your trunk and interior. You could take your spare out of your trunk and drive around with a can of Fix-A-Flat. Lighter wheels will improve performance and mileage, but if you have factory alloy wheels, anything lighter that fits your car will be very expensive.

You will find more precise information on your car if you search for a TL forum. Stock Honda products are tuned pretty well for performance and efficiency, so there probably isn't much room for improvement that won't cost a lot of money, but a group of TL owners would know better than I.

If you own this car, the most effective way to improve fuel efficiency would be to sell it and buy something smaller.