Although your question is very broad, it is fun to think about.
There are so many possibilities we could keep this thread going for a long time.
Especially if we include both chemistry and physics.
Some if my initial thoughts were making soap, making bio-diesel, and creating dyes.
Maybe you can get some inspiration from this site
http://www.cavemanchemistry.com Fire
Stone Tools
Unit Factor Analysis
Fermentation
Ceramics
Textiles
Metathesis Reactions
Alkali
Metals
Lime and Plaster
Redox Reactions
Dyes
Glass and Glazes
Paper
Stoichiometry
Alcohol
Acid
Soap
Soda
Batteries
Aniline Dyes
Photography
Ammonia
Electrochemicals
Pharmaceuticals
Fertilizers
Plastics