+1 on Trex, especially if you want durability outside and exposed to the elements.
The material will differ greatly from person to person. A high-end marine dealer might choose to use Teak, where that 1"x4"x8' runs about $100. A garage sale would use marking pen on a paper sign. It's very much how much you want to spend and what look you want to create.
The same is true with plaques. If it was something to be kept long-term and of great value (worth spending lots on), I'd use premium wood, solid brass for all finishing and screws, etc. If it was a something with a limited life expectancy, I'd look at something less expensive.
The tradeoff is a bit of time vs. money too. If you buy pine or poplar and stain it to the color of walnut its another step. You lose time but spend more money. For small plaques (to be inside) I'd locate your local hardwood distributor. You may well be able to buy a short piece of hardwood (i.e. 1x4x10") for a few dollars.
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