When I was in Kenya last year, we did things like leave behind cheap T shirts (which we'd worn and didn't want to take home with us) in our lodges with a note giving permission for the room attendant etc. to keep it if they wanted it. Same with half-used toiletries etc.
Would the same thing be gratefully received in Tanzania?
Also, our itinerary includes a visit to Mto wa mbu, is there anything I should take for the people there? (When I visited the Samburu village I was so cross with myself for not taking any childrens clothes when the kids were running around in literally rags).