Our green contributions tend to be reusing things and/or buying used, so not all my suggestions will fit the 'green' category. However, these are all really good open-ended toys.
Playmobil sets. Dd's playmobil doll house is a favorite toy with all the kids who come over (ages 5-9 ish).
Realistic or real dress-up items/props. One simple item is a lanyard with an "ID" on it. Lanyards/IDs turn out to be crucial props for playing school, hotel or any other role playing game where the figures might wear an ID. If you add a whistle, you're a teacher. Keys, and you're a security guard. (You can use old keys or go to the hardware store and ask for blanks that they miskeyed.) Real uniforms, real IDs, real work tools are a big hit.
Small white boards with markers -- again, this is the kind of open ended toy that can be used for many different things - from 'teaching' to announcing sales at your shop.
Play kitchen - still used at our house for kids ranging from 3-8. Toy food helps too.
Tree swing -- hands down the most favorite outdoor toy we have. Even the big kids come to play on it!
Board games of all sorts - Kids on Stage is my favorite for kids 4-6. Cranium Cadoo is ds' current favorite (he's 8).This catapult
is a really cool toy and has been used to catapult an amazing array of things.
Dd is starting to outgrow it, but this doll stroller
from Nova Natural is great. Sturdy, rolls well and fun.
A shopping cart is another good toy that kids love. Again, it's amazing what they use it for.