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):

Additive/desktop Manufacturing: Edit

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.

Airships: Edit

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. 

Architecture Edit

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:

  1. Real world projects (1 Bligh St Sydney, Ole Scheeren's work, ArchiBlox's carbon positive prefabs)
  2. Speculative projects

Arcologies:  Edit

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 Edit

Will the development of more advanced robotics and AI result in technological unemployment?

Floating Cities: Edit

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.

Green Engineering: Edit

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. 

Hyperloop: Edit

Elon Musk's high speed vacuum tube transport system looks to be an interesting way of going from city to city.

Maglev trains: Edit

Another option for town to town transport is maglev trains.

Nanotech: Edit

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.

Permaculture: Edit

If we don't make the mess in the first place, we don't need to clean it up.

Velomobiles: Edit

Streamliner recumbent bicycles, these let people travel at car-like speeds without using fossil fuels.

Warp Drive:  Edit

The White/Alcubierre concept opens the possibility of practical interstellar travel.  Anybody want to go next-door and meet the neighbors?