You might consider the Pacific Northwest - Vancouver Island/Vancouver area, or a bit south the Seattle/Olympic National Park/Cascade Range, or south of that the Portland/Mt. Hood/Oregon Coast/Bend loop. All of those should give you some beautiful mountains/oceans/lakes although not really magnificent fall colors like the east coast. No heat or beach holiday type experience.

Perhaps San Francisco/Lake Tahoe?

Since you've been to Banff, I won't suggest the Colorado Rockies around Rocky Mountain Nat'l Park; too similar. But you could try Durango, which is just north of New Mexico, just west of Mesa Verde, and right at the start of the the magnificent mountains. You could catch a variety of landscapes as well as the aspen color and mountains. The Durango Railroad could take you on a day trip up to Silverton.

I expect others to come in with some wonderful East Coast options for leaf peeping..

Newfoundland and PEI

In October, Niagara Falls and the Niagara Region are alive with Color change, harvest festivals and plenty of scenic drives... you could head north to Toronto and beyond, or South through the amazing Finger Lakes in New York.

any crossing on BC ferries becomes magical as one approaches the islands. all you need to do is walk from one side of the ferry to the other and back again
cheers
AndrewDavid

Vancouver Island's west coast: Tofino, Ucluelet, Pacific Rim National Park. Stay somewhere like the Long Beach Lodge Resort:

http://www.longbeachlodgeresort.com/

The Gulf Islands: Galiano, Saltspring, Mayne, Saturna, Pender.

Vancouver / Whistler / the Sunshine Coast (especially the West Coast Wilderness Lodge): http://wcwl.com/

... and I should mention, the beaches here aren't "beach holiday" style places where you sunbath and slather on the sunscreen. Rather, these coastal areas are famous for their awe-inspiring scenery, rainforests, and wildlife - bear-viewing, whale watching, tidal pools rich with marine life.

Here's the aerial photo of the Tofino area (next to Pacific Rim National Park): http://www.islandhavens.ca/Tofino%202.jpg

Newfoundland.

I agree.. Do consider Newfoundland. You won't be sorry.

I'm surprised that Nova Scotia's Cape Breton Highlands and the Cabot Trail have not been mentioned (unless I missed it). One of North America's most famous scenic drives and it is also a very popular area for leaf-peaping, although I don't know whether your timing will coincide with peak colour. Both Celtic (Ceilidh Trail) and Acadian (e.g. Cheticamp) culture on Cape Breton.

You could start out in Halifax as your "city" that you'd like to explore - it's a very nice small-to-medium city with a Harbour District offering Market, Museums, restaurants, pubs and shops. Also the Historic Citadel.

That would leave you with 11 to 14 days, more than you need for Cape Breton (I would allow minimum 3 and up to 7 if you want to really explore the nooks and crannies). So, after Halifax you might tour down the charming and scenic South Shore (Lighthouse Route) southwest of Halifax, visiting historic towns including Lunenburg, scenic little Peggy's Cove, Mahone Bay, Shelburne and enjoying some beautiful coastal vistas along the way (get off the highway and hug the shoreline as much as possible - there are good mapbooks available). Then up to also charming and historic Annapolis Royal after which you could take the ferry from Digby (famous for Scallop fishery) to Saint John New Brunswick and from there spend a couple of days touring the awesome spectacle of the Bay of Fundy Tides, with great viewing at Alma/Fundy National Park and Hopewell Rocks. Then Cape Breton and back to Halifax for your flight out.

Actually, you might want to do this in reverse - research timing for leaf colour (variable year to year but aim for the average peak) and try to time your Cape Breton visit as close to that as possible. Others on Fodors who have visited at that time can probably give some guidance too (we've visited many times, but never in fall).

Actually, I wanted to write cape Breton and Newfoundland. I just like one word posts.

... and the thing is to visit both Cape Breton and Newfoundland, you have to take two long ferries or rent a car one way. If I had to choose, I'd go with Newfoundland.

i dont know why i dont hear recommendations for palm springs...its such a different type of place to experience so id do that if youre landing in LA..not a big detour and just something really spectacular

i loved the national park there up in the mountains...loved the butterscotch bark trees and the views...the town itself is gorgeous

Good point, lanejohann. Although I didn't think of it because it may be still way too hot for them in October.

In late March this year, we flew into San Diego, drove out to Yuma, Arizona, for a couple of days (visiting a cousin wintering over), then a week in San Diego area, ending with a few days in Palm Springs (La Quinta Resort, actually). We found Joshua Tree National Park fascinating. And in Palm Springs area, the views of those stark mountains were beautiful, plus the weather was sunny and warm but not hot.

Hi guys.
I just want to thank you all for your suggestions / replies. Since my post I’ve spent the vast majority of time looking at all suggestions and now feel as though I’ve been to them already! I’ve worked out flights from UK, internal US flights, things to do, mileages, costs, etc. etc etc. Yes, I know, my kids have already told me I’m ‘sad’ but I love it. Anyway, I’m about to start a new post with reference to our choice (unless 'er indoors changes it again) and felt I ought to offer my thanks.
It SEEMS the awe inspiring scenery will have to wait as the wife has always wanted to go to New England in ‘The Fall’. (What a lovely expression for Autumn!) (What a shame the wife hadn’t said that in the first place!!)
As we have to arrive in mid-October (for 16 days) it seems we’ll miss the best of Vermont and New Hampshire and as the good lady wants to go to Niagara Falls (Canada) too I’m not sure of the best route / flight etc yet or how customs work for a Brit crossing USA/Canada border.
I digress; a very big Thank You to all who’ve helped.
All the best for the New Year !!

I would be tempted to fly into Boston and drive to Acadia National Park for fall foliage. I think it comes a week or two later than the rest in the area. I think around Oct. 10 would be optimal, but is different each year. From there you could head over to Niagara Falls. Perhaps, even the drive through NY State might have a lot of fall color too, I'm not sure.

Another option would be to start in DC and see Shenandoah National Park for fall foliage, then through Pennsylvania to Niagra