Easy and fun to build

Five things you could start this weekend. Ordinary tools, no specialist skills, and each one does something visible on the first day.

Solar cooker: Zero-fuel cooking on sunny days; output timing depends entirely on weather. — Why this one: Works the first sunny afternoon you try it.
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: A box and some insulation. The physics does the rest.
Solar dryer (dehydration): Direct or indirect solar dehydration of fruit, vegetables, and herbs, removes enough moisture to halt microbial growth, extending shelf life from days to months — Why this one: Cheap to try, and the result keeps for months.
Bio-sand filter: Slow sand filtration with a living biological layer at the surface; effective and needs no electricity, but requires a maturation period and careful maintenance — Why this one: Easy and potentially life-saving.
Bokashi fermentation composting: Anaerobic fermentation in a sealed bin; handles cooked food/meat scraps that open composting struggles with. — Why this one: Fits under a sink and needs no garden.

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.