1 - If you are sitting in a window seat and see a black triangle symbol directly above your seat, congratulations:
you are sitting in the "William Shatner seat".
The triangle is simply a visual marker to identify the window/seat that offers the best possible view of the wing.
2 - If you notice a small button near the lever of your tray table, congratulations!
You're sitting in the "Overwing Emergency Exit Row." This row of seats is found, for example, on the A320 and most other smaller aircraft. The button prevents the tray table lever from folding into the open position when passengers squeeze between these seats during an evacuation to reach the Overwing Emergency Exit.
It prevents the tray table from folding down and blocking the way to the exit.
It also serves as a visual and physical identifier for the emergency exit when the cabin is dark and filled with thick smoke.
The other seats do not have such a button:
3 - Country of origin and aircraft registration number.
This information is clearly displayed on the side of the aircraft.
Note this number and search for it online. You'll then find out a lot about the aircraft – from its current status (via the flight tracker), to planned routes, previous flights, accident history, leasing information, where the aircraft is based, and so on.
Even with smaller jets, private planes, and older aircraft like the Boeing 747/757, you can find a wealth of interesting information this way.
And it's a great feeling to discover, based on the registration number, that you're flying on the exact same aircraft as on your trip last year!