Can I do this in my flat?

No garden, no roof access, and a landlord. Most of this catalogue assumes a house; these are the options that do not.

Bokashi fermentation composting: Anaerobic fermentation in a sealed bin; handles cooked food/meat scraps that open composting struggles with. — Why this one: Odourless, sealed, and sized for a kitchen cupboard.
Haybox / retained-heat cooker (fireless cooker): Food is brought to a full boil on any heat source, then transferred, still in its pot, into a heavily insulated box (traditionally hay-packed, now often reflect — Why this one: Needs a hob for ten minutes and nothing else.
DIY activated-charcoal air filter: A bottle or box packed with activated charcoal, run passively (no fan) to adsorb airborne particulates and some VOCs from indoor air, a cheap complement to vent — Why this one: Healthier interior after a couple hours' work.
Insulated draft-blocking curtains: DIY window curtains built from an outer fabric layer and an inner reflective/insulating layer (e.g. mylar or foam-backed material), sealed to the window frame w — Why this one: An easy way to stay warmer for cheaper in winter.

This is a short list, not the whole catalogue

These are a handful of picks to get started. The full set of options for every need is in the catalogue, compared on the same axes and filterable.