Healthy obsessions are those which generate and preserve value, rather than ride on the presumed value of the market system, hollywood or someone else.
Most hardware is extremely dank, representing enormous, ordered complexity that is not easy to duplicate. Something on the order of 105 (Intel 8088) x (210) (memory, I/O subsystems, video graphics/audio) ~= 109 for the original IBM PC. So, even that is a great find, no matter how much the market will tell you how obsolete it is. Technical Reference manuals for such is like striking gold. They've got complete ROM BIOS disassembly, electronics schematics, I/O interface specs, and more.
- math: arithmetic, number theory, fractals, geometry. Just say "NO" to about anything in topology, the bastard step-child of geometry.
- radio engineering: HAM radio hardware, sophisticated antennas, microphones..?
- computer engineering: Personal or epic minicomputer hardware, some peripherals are also very valuable (IBM dials & buttons, e.g.)
- Textiles: How to make your own cloths, customizing siad cloths, making clothes, sewing and stitching techniques.
- Readables: Bookmaking, papermaking, screenprinting, logo design.
- Music: Learning to master an instruments, writing music, mastering the masters, ensembles.
- Industrial Design: Good product design, UI, packaging, power efficiency.
- electrical engineering: Building circuits, reading schematics, hardware and their ordering, interesting functions
- civil engineering: Good building design, urban architecture, light-footprint transport networks, signage.
- technical writing: reference manuals for all, good technical writing is an immense benefit for all.
- software engineering: classic software collections, diagnostic aids, text games, science and engineering-related, simulation
- audio engineering: analog synthesizers or digital ones with open source software, multi-track recording equipment, good microphones and speaker design.
- old-school gaming: working, old-school arcade games with owner's manuals, ROM-level emulation on old console games, D&D.
- mechanical Engineering: machining and milling, complex gear assemblies, legos, extreme alloys, pneumatics.
- geology, metallurgy: both metal and mineral-based crystals, superb examples of these are quite impressive.
- minimalism: you always have some kind of weight and size capacity as a personal constraint when acting in the world, what will you keep around to maximize your utility? Some survivalism can be healthy.
- naturism: Most of life has to be considered one of the first great organizers of order. All the great trees, colorful birds, furry mammals, flowers, and butterflies -- sacred and not to be molested with. Order is not the natural state of the universe. Admire it from your greatest instrument: your own eyes. Before you try to force value out of that shared resource, remember patience and respect ultimately reveals solutions. Find the Tree of Life and develop a real society.