The absence of iconography is important. It's too easy to make something appear to be 1980s by shoving a Rubik's Cube into the scene, or a Space Hopper for the 1970s. I want the environment to feel right before adding stuff like that into the scene. Hopefully, the cumulative effect will counter any adjustments made to items (where these items might otherwise infringe copyright or trademarks).
The 1990s has been a hard decade to pin down in this manner. However, with help from classmates, the first aspect to address was identified as the use of colour & patterns.
Dayglo and fluorescent colours were all the rage in fashion, mostly fuelled by the clubbing and rave subculture of the early '90s. Ultraviolet "black light" causes these colours to glow, and this effect helped create a "modern" spin on retro-1960s psychedelic stylings.
Patterns included animal (leopardskin spots, zebra stripes) and patchwork.
Another innovation of the period which leaked into fashion was fractal mathematics, which had surged in popularity during the late 1980s with advances in home computer technology & graphics. In line with the aforementioned 1960s retro affectation, Mandelbrot patterns became the new Paisley pattern equivalent.
The 1960s "vibe" was not only a cyclical fashion trend but also served as a symbolic link for rave drug culture, with fractals and dayglo colours fitting neatly into the psychotropic framework.
Room decor changed as well, but in a different way. Where the 1970s & 1980s had focused on particular colours, the 1990s was a bit of a free-for-all (before returning to 1970s browns in the first decade of the 2000s). Eclectic mixtures of pattern and colour became acceptable.
Of course, like the preceding 1980s, the whole decade did not follow a single trend. However, the highlighted visual indicators serve to drop the viewer into a time period quickly.