I believe that the crayfish is mostly a meat eater. Yes, they are omniverous, but I believe they prefer meat. At least when ever I have cought them it was sith a piece of meat tied to the end of a string (as a kid) I would assume that if you keep the littel guy satisfied as far as food goes he probablt wont go after your plants. I would try some of the tougher varieties as others have already mentoned. Java ferns, moss on rock or wood and maybe some anubias species. These are all plants that you will not have directle in the substrate so the may stand a better chance if he decides to forage a bit. Some floating lpants would look nice to. Hornwort (Certophylum submersom) has already been mentoned and is a nice looking plant and a wonderful nutrient sponge. It also seems to make some epaphyte (correct word and spelling?) proberties which means that it secretes a chemical into its surroundings that inhibits the growth of other things. In this case it seems to have that effect on algae. Other plants do not seem to be affected bby it. Hydrocotyle leucophalia is another great floating plant that looks nice planted as well. Sylvania is a beautiful floater too. Elodea is interesting, easy to find and a good nutrient sponge also, plus, it would be biotypical for your crayfish.
I have a suggestion that might save you soem money. Actually 2. First, it seems that you like to go out searching in nature to find things. I do aswell, but thats another story. You could find some nice plants inthe late spring early summer but that is a bit to wait. You can also try those Pet???? places. They ofter have decient plants at cheaper prices and often you can find something neat that got thrown in a mis-id'ed for a very cheap price. If you hit them right after they get a shipment of p[lants they are usually in pretty good shape. I have found that to be a great way of finding anubias sp. They are a tough plant to so mabey thats why you can find them. Often an anubias from Pet??? is only $3 and it is almost as nice as the $6-7 one from the lfs.
Any way I ramble, so luck to you.