Under this lens we have endless examples of answers to your question today. Hobbyists with CNC machines, high school students sequencing DNA, satellites and drones monitoring for forest fires, a good portion of HN posts about open source hobby projects, all the art people produce, etc.

We also have a lot of sci-fi which presents many ideas, some of which may come true, some of which have come true, and some which will probably never happen.

But my guess is this is all missing the spirit of your question, and I don't want to come off as curmudgeon, so I will do my best to answer in good faith:

If we truly have abundance and all needs are met, and there is no longer a need to be productive:

- I am a curious person so I will say experiments to advance understanding. Building more and more complex simulations to understand illusive aspects of reality.

- I am a creative person so I will say art, more impressive and expansive installations which serve no purpose other than to express and inspire.

- and I am a realist, and although it is not a motivation if mine, I expect it always will be of some others, so I will say social status. Ever more weird and complex ways to infer social status.

In other words, if we no longer need to engineer for economic productivity then perhaps we will engineer more for things that are not directly economic productive.

But I think it's more likely that the free energy principle applies at all levels of abstraction and we will always have some forcing function that biases resources towards increasing economic productivity, with some small percentage (with ever increasing absolute magnitude) left over to play with.

Money is just a form of potential energy for getting jobs done, and the more productive we are and the more money we have the more ambitious jobs people will want to get done.

Building stuff is about solving for scarcity.

In a post-scarcity world (and I realize it is more of a gradient than a binary), what reason would there be to build things other than for amusement?