Since you are in construction, may I suggest woodwork.(?) I've seen some amazing items made from discarded shipping pallets. These pallets are usually free. For ideas on what to make google pallet woodwork ideas.
caution :There's soooo many ideas.
In 2017 I am going to Malawi to build classrooms. In order to get there I need to raise £2300. To raise some of this i was looking to make and sell things. I am looking for ideas on what I could make easily to then sell on to help raise funds?