The scientist in your life has a specific relationship with the world. They look at things longer than other people. They want to know how processes work, not just that they work. They find the phrase "but why?" completely natural in any conversation.
Most scientist gifts fail because they treat science as a subject rather than as a way of thinking. The periodic table mug, the DNA helix keyring - these acknowledge science as an identity without engaging with any of it.
Axisophy makes prints for people who actually think this way. Mathematical structures computed to high resolution. Scientific insect illustrations grounded in entomological reference. Orbital photography of Mars. Here are the best options by scientific discipline.