agree!
http://www.allposters.com/-sp/Redwood-Forest-Posters_i8664905_.htm

Take a trip to The Legacy Gallery in Jackson Hole and Brad and Ginger will show you the perfect piece of art. Or, the Trailside Gallery.

These suggestions may be cliche but \I'll go ahead anyways ;-)
Rustic clock
old skies
snow shoes
canoe
paddles

If this is a family getaway, why not have a professional photographer go along on your next family ski outing and take photos of all of you in the snow? Have it professionally framed and matted to be just the width of the firebox and hang it. If you like the outcome, hire him/her again in the summer for a swim outing to get pix for the warm season, and change them out twice a year.

It looks like the space above the mantle is rather large, so you need to be conscious of scale. I'm not sure if you can get a poster the size it needs to be, and not sure a poster is the rustic that would fit in with the existing rustic space. I live in a small log cabin in northern Michigan. Above my fireplace is an oil painting of the Teton Mountain range. I find the subject rendered in oil paint fits the rustic space to a tee.

How about moose antlers or fish taxidermy

Go elk hunting
Elk antlers can be approx 5' in length and they are quite full

Wow! Thanks so much for all the great suggestions! My original hope was to hang wooden canoe upside down but hard to find the right length (8 foot) Love the idea of oil painting…hoping for something warm (in reds, browns, golds) Also thought about "rug" or tapestry wall hanging as possibility too. I will try suggested Jackson Hole gallery. Also thought about a series of crests? Any ideas for rug like wall hangings?

lovey123: Perhaps something like a vintage Pendleton blanket would work. There are so many beautiful ones, many with meaningful stories. Antlers above, blanket on iron rod in center, large iron candlesticks on either side.

Art in the colors you want in the room, an image you both really love . . procure it on a meaningful trip to memorialize your travel. You will know it when you see it . . . the canoe can go into the rafters when you find it . .

Lovely123, have you thought of hanging a quilt? Lap quilts are great and light and a fraction of the cost of a large painting. I can custom make one for you if you have something in mind. Or check my shop, citrusquilts, at etsy.com. A custom made lap quilt using fabrics from clothes worn by your family members would be great!

A large pinecone wreath might be nice. I like the idea of a lap quilt, too, maybe hung on a fairly straight branch of wood.

Navajo blanket or rug would be fabulous.

I have the perfect canoe for you. 5 foot display canoe will fit perfect.