You can't beat a 12 gauge pump for versatility and reliability. There are many different loads out there to increase your options, including signal flares (rare). A basic 22 is highly useful and perfect for younger folks.

If you get something in the shot shell, centerfire category, you might want to consider handloading ammo. Using less extravagant equipment, it is very cheap, a fascinating pastime in its own right, and you can increase the range and versatility (there's that word, again) of your weapon.