If you're looking for a high quality, decorative steel grill for this area, we often recommend Steel Crest (http://www.houzz.com/pro/steelcrest/steelcrest-architectural-accents). They are a manufacturer based in Arizona that creates custom HVAC grills and registers, among other things.
For a large floor return grill like this, there are 17 unique faceplate designs and 14 powder coat finishes available to select from in the Gold Series. They also offer the Pro Linear and Pro Vertical designs if you are looking for a more traditional pattern. You would probably want to updgrade to 1/4" thick steel with beveled edges for a piece this large. They also offer concealment louvers as an accessory which can be added to the grill to help prevent guests from seeing the underlying duct work. You can take a look at their design options on their website: http://www.steelcrestonline.com/hvac-registers/gold/
Steel Crest offers some of the highest quality decorative grills that we have available. While these pieces are both beautiful and durable, they are custom made and typically subject to a 5-8 week lead time to be manufactured. For a 30" x 14" Floor Return including an upgrade to 1/4" thick steel and beveled edges, the price comes out to $659.09. While this is quite an investment to make in a floor grill, you also "get what you pay for". These grills are made to last a lifetime.
If you're looking for a more economical solution, you may be able to install a filter or a screen below the grill. It seems that part of the concern is the tidiness of the duct work below the grill and guests that may be able to see into the duct. You could purchase a large "hog hair" air filter, cut that to 30" x 14", remove the grill, and set the filter flush in the duct to help prevent visitors from seeing this area.
We would not recommend a wooden grill as a replacement for an opening as large as 30" x 14". The main concern is the structural integrity of the grill. If you decide to go with wood here, it is very unlikely that it will hold up to foot traffic or furniture being placed over the grill. Just keep that in mind when you are making your decision. Good luck!
Q