You can organize a function in which include some motivational speakers, invite folks those got benefits from your NGO they can mention how the workers help them, this will motivates others also and appreciation will boost the morale as well. Organize games like (Cricket Match, Build the Tower, Balloon Games ) so team get engaged and get the chance to know each other. You can plan for some gifts as well.
I may add, Are planning an outreach, picnic type, stay-get-together or within the four walls of your office?. Is't going to be totally recreational or partly this partly to get to know works of others.
One inevitable task going to be, considering your NPO activities, the teams coming from diff.centres should share their projects' nature, target groups, activities, achievements, misses and the troubles/bottlenecks and ambitious focus. They might as well highlight help & guidance expecting from other groups. Also should hint what other projects they could take up in the future either with collaboration of other teams or pass on to other specialised teams.
Secondly, I understand, lots & lots of aid, support, grants just gets returned to govt.or other sponsoring agencies, national & international, especially in the field of education, rural health & nutritional, environment, waste management, organic farming, rural finances etc. your teams should focus on such projects for the benefit of target groups, if not earlier.
Thirdly, thousands of tribal/rural population still remains in the country to whom even electricity, communication, health & education, road etc have not reached them. To start with, they should be contacted, provide electricity atleast with local based solar power stream, associated with safe drinking water and minimum health requirement. Train them in applied research in agriculture & safe, healthy living style, to eradicate disease passed from ages from generation to generation etc.
Reg.recreational side, there are enough sites which provide various games to entertain all.