You don't want liquid nitrogen you want nitrogen gas 2 different things. Liquid nitrogen is used to super cool things, they put nitrogen gas in tractor tires because it doesn't freeze like air, very little moisture.
nitrogen in a gaseous form is used in aircraft tires, for 1. since the molecules are larger the pressure doesn't bleed down as quickly as air, 2. no moisture (water vapor) in nitrogen to freeze at altitude or corrode/errode the metal and rubber parts. nitrogen in liquid form is usually used to supercool parts to shrink them, like installing bearing and bushing. it's also used in cryo-treating of parts after heat treating, but you're going to have to do your own research on that. nitrogen once mixed with another gas will not seperate into layers, ex. check the air outside.
Take a seal-a-meal bag put some liquid nitrogen in it seal the opening and throw it under your buddies tool box, stand back and watch him jump, lol. Take a 2ltr plastic bottle put a rubber stopper in the opening with a hole in the stopper for a tube to fit in the hole, fill it 1/4 way up with water, pour some liquid nitrogen it and air it up with an air chuck, takes off like a rocket.