There are a lot of areas of engineering that could help create the SolarPunk future we'd like to see. Just a few from off the top of my head (Joel, 9/25/15):
If you can print your widget out yourself, you don't need to get the whole supply-chain involved. Technologies that fit this include 3-D printing and desktop CNCs.
For intermediate distances (say, San Francisco to Seattle, or anywhere-in-England to anywhere-else-in-England) a modern Zeppelin can get you there at least as fast as a jet airliner for vastly less fuel.
Our constructed reality, the boxes and walls and roofs that we all live nad work in most days. I (5tephe) can see this topic being divided up into:
- Real world projects (1 Bligh St Sydney, Ole Scheeren's work, ArchiBlox's carbon positive prefabs)
- Speculative projects
The 'city in a box' first invented by Paolo Soleri. There are hordes of sub-technologies that can go into these, such as active-solar windows, silent wind-generators, carbon-fiber structure (much lighter and stronger than concrete), vertical farming, indoor forests, and so on.
Artificial Intelligence & Robotics
Will the development of more advanced robotics and AI result in technological unemployment?
One way to put people where we need to be. Here is an example.
The Seasteading Institute is currently doing a lot of work on this.
There are a lot of simple/easy things that can be done to reduce environmental impact -- insulating our houses, using LED lights instead of incandescents, using more energy-efficient methods of transportation, living in smaller houses, and so on.
Elon Musk's high speed vacuum tube transport system looks to be an interesting way of going from city to city.
Another option for town to town transport is maglev trains.
While we have been aware of the dangers of 'grey goo' since nano was first thought of, its potential to augment human experience (for example, drug-delivery systems for medical care) means there's too much there to not take a look at.
If we don't make the mess in the first place, we don't need to clean it up.
Streamliner recumbent bicycles, these let people travel at car-like speeds without using fossil fuels.
Warp Drive: 
The White/Alcubierre concept opens the possibility of practical interstellar travel. Anybody want to go next-door and meet the neighbors?