Wow, no one has mentioned a bicycle, I got mine for around 300usd like 10 years ago, a simple but useful MTB, since then I have invested no more than 800usd in addons and repairs ( to fix a bike is so cheap!) With it I've traveled for around 15 cities in my country (Colombia), I've discovered amazing places: lakes, forrests, mountains, rivers, towns and more... I've met lots of new friends and saved tons of money as it became my main mode of transportation, it also allowed me to know my city: new parks, new cafes, new spots you can't really see from a window's car perspective. And additionally, it allows me to control my weight without having to spend a cent in a gym!