I'm a big fan of huge rocks. Very effective. There are a lot of them. Highly entertaining on YouTube.

100% effective at reducing people driving over the edge or corners of property.

> or unusually sturdy art installations

That would be great! I'm imagining a hypothetical future where the city partners with local artists to produce bollards in disguise, each one unique and one of a kind piece of art.

The last ones are, in fact, bollards.

Stones can be also a form of bollard.

You could also turn bollards into art installation, which goes back to first line ;)

How about guerilla gardening, except complete with setting up stone planters, the ones that probably weigh far north of 100 kg? I wonder what would happen if someone were to set up such a planter in the spot where a bollard ought to exist?

There's a great, possibly not well explored, area of "how the fuck do we move that planter stealthily".

I suggest hi-vis vests, helmets, and appearing like you're in the right place just doing your job.