I have been to Portland numerous times, always do some photography while there. The one day I went to the Head Light, it was foggy, but I still got some decent images. Fog tends to keep moving on the coast, so you can still capture images when there is a "hole" in it! Same thing happened one of the times I went to Nubble in York. As long as the weather cooperates, you should be able to get some good photographs. There will be other people there, but patience will make it possible to keep their presence in your images at a minimum. There is not a lot of space to move around the base of the light, which puts a lot of people out of sight behind it! You can take photos from various viewing points on the shore, too. The 18-135 would be good for getting a closer view from a distance.

Don't ignore the Old Port. I like to walk out on a long wharf and take photographs of the port and city. One I got right before sunset that is spectacular! You will find an interesting mix of restaurants and warehouses, working boats, pleasure boats, and ferries. There are even seals that hang around!
There are a lot of shops and restaurants on the land side of the road, making it desirable for those that want to shop and eat. But the look of the restaurants on the wharf side, both on the road and on the wharfs, is much more interesting photographically.

It is hard to "do it all" in one afternoon, but not impossible. Pick your spots and have fun! You may even want to go other places in Maine - lots to see!

Look at my photos page to see some examples. 500px.com/susanfromvermont/galleries