The simplest:
The Mark II:
If you put a smooth round tube in moving fluid, it will generate a street vortex:
It can be a string and the force will vibrate the string. At each end there will be vibrating pull.
It can be a tube, the tube will experience vibrating force.
It can be cables on a suspension bridge and at least one big bridge fell apart because of it.
I understand that at least one car company tested piezos inside a car muffler and generated useful current so that the alternator had to work less, thus saving gas.
Faster vibrations are better. You only need a source. The less moving parts like axles and bearings to maintain, the better. Suppose it is raining hard and you seal the unit into a card or plate? Would a monsoon just hold it down unless it was small?
Piezo disc here costs $1. How long to generate $1 worth of electric?
The US Army experimented with piezos in boot soles to help charge batteries. They did, but they also slowed the soldiers down just a little in tests. Conservation of energy at work, no free energy. A road or sidewalk would have to take away from what travels on it.