Steal a plane that's equipped with a good GPS system (say a Garmin GNS-430 or GNC-530) coupled to a 3-axis autopilot like the STEC Model 55, pack the plane with fuel and explosives, then bail out before impact. Your only cost would be the explosives and you could probably steal them like the plane.

Most planes are very easy to steal and the security at most small airports is lousy. You could easily find a well equipped plane with good load hauling ability like a Cessna 210 at hundreds of airports around the country. It wouldn't be all that difficult to bail out of one, either. If survival isn't an issue, then your options become much larger. A big old plane like a Twin Beech (Model 18) can carry about 3000 pounds of fuel and explosives at around 200 MPH. You could do a lot of damage with one of those and there are probably several hundred of them scattered around the country (it was in production for over 30 years).