Resistance Band Set with Handles
Fits in a drawer, works in a room with no floor space, needs no gym. If you are starting from nothing this is the highest-value thing in the category, and it travels home for winter break.

The splurge, and it replaces an entire rack. One warning that never makes the listing: if you live above the ground floor, dropping these is a conversation with your neighbours. Budget for a floor mat too.
Fits in a drawer, works in a room with no floor space, needs no gym. If you are starting from nothing this is the highest-value thing in the category, and it travels home for winter break.
Thickness is the real specification. Thicker is kinder to knees and elbows; thinner keeps you stable in anything balanced on one leg. Pick for what you actually do, not for what looks comfortable.
Get the hollow-core one. Solid foam rollers compress into a flat, sad log within a semester of being sat on, stood on, and used as a doorstop.