What would be my target?
Something small scale, like a school?
Then I would simply activate the fire alarm, stand where the students are set to assemble and use some frag grenades. Mission accomplished, exit strategy depends on the venue.
The assassination of a high-ranking member of the public?
Aquire certain proteins that regulate bodily functions, mix them with a certain set of chemicals that enables them to traverse the skin, put on a realistic-looking set of latex-skin gloves, apply solution to hands, shake targets hand, watch as he dies of "natural causes" a day/few days later.
(Note: Staying vague so as not to give anyone ideas, also, aquiring said chemicals is not easy if you aren't working in a lab that has a reason to deal with that kind of stuff9
Taking down a plane (no hijacking, simple blowing it up):
First: You need an elderly fanatic. A grandmother e.g., have her undergo hip-replacement surgery, fill her femur with plastic explosives and a special hip prothesis that doubles as a detonator, which, in turn, is activated by a remote-control disguised as an insulin-pump.
Nervousness in the elderly is usually attributed to fear of flying, and, in that specific case, can be attributed to pain as well, since he/she just had a major surgery. Then you get her a window seat, preferrably over the wing and boom goes the grandma.
Another very effective way to go, especially in an urban environment would be a so-called "suitcase nuke" or several of them. They are extremely hard to come by, but, since I am not vested in real-world black-market affairs, which I would have to be in order to become a succesful terrorist, I leave the acquisiton to you.
This suitcase nuke you place in a crowded space in a hard-to-access spot, possibly close to a famous landmark. Get far enough away and have it detonate, make sure to use both, a timer and a remote-control, whilst ensuring that you do not send threats before the act but rather take credit afterwards.
That would be my first take on this topic, maybe I come up with more later.
One more:
Detonate a powerfull EMP ins a major commerce-center and watch the global economy spiral downwards. (Recommended target: Wall Street or Silicon Valley)