What are easy ways to increase a car's horsepower/performance?

The easiest and cheapest way it to increase air flow...get a high performance air intake system (K and N makes very good products) and then a good high flow exhaust (Flowmaster, etc.). You can also purchase a plug in chip that can change your engine dynamics. They can get a little pricy but it is easy to install. Other than that, you are looking at major dollars for what you want. Turbos, superchargers, etc. can boost performance but you are looking at a lot of money.