Ultra-Lights, Small fabric skinned aircraft powered by engines comparable to large chain-saw engines or small snow-mobile engines. Cost; for an outright purchase would be (guessing) $5,000-$10,000. Could carry a payload of a "few" hundred pounds.
Hot Air Balloons, Cost; similar to ultra-lights, with a greater payload. Balloon ride could be rented for a nominal fee of $100-$200. The pilot could be taken out soon after launch...
Para-Planes, Motorized para-chute with a propeller driven by a small engine comparable to an ultra-light. Cost; (guessing) under $5,000. Payload of a couple hundred pounds at most.
Para-Chutes, Limited range (depending on exit altitude and winds aloft), limited payload (200lbs. approx.). Cost; under $5,000. Would need a means to reach altitude. Hot air balloon, helium balloon (see below), aircraft, etc. Would provide stealth arrival on target (freefall).
Hang Gliders, Limited range and payload. Low cost. Rentals available. Considering the above examples, this method would be one of the least desireable.
Sail Planes, Limited range and payload. Low cost. Rentals available. Some stealth qualities.
Helium Balloons, This is probably the hardest to detect and stop. Weather balloons (5' approx.) combined for the proper lifting capability. Low cost.
A truck driver actually pulled off a flight with several weather balloons attached to a lawn chair !!! He had a BB gun that he used to initiate a descent by shooting holes in the balloons. Commercial pilots spotted him at 17,000' over Los Angeles in their air corridor.
If terrorists want to terrorize the USA, there are numerous ways to get it done. I have just thought up several aerial applications. Others may arrive at different methods of aerial application. Surely the terrorists can do likewise. The evil doers have shown us that their commitment levels are high. Where there is a will there is a way. Our country has to become far more vigilant in detecting terrorists BEFORE they enter our borders.
PROFILE ! PROFILE ! PROFILE !