First let me say that you got an AWESOME deal!
I wouldn't bother with an airstone, but I would definitely run a filter and cycle the tank (I strongly recommend a fishless cycle using pure ammonia), as you will definitely not enjoy doing 100% water changes on a 10 gallon! Clean water and stable parameters are the best gift you can give a fish :)
To prevent fin damage, I would wrap the filter intake tube with some nylon or slide a sponge over it (you cut a hole into a piece of sponge, then slide the tube into it). Also, you may want to consider baffling the outflow area depending on the strength of the current. As for decorations, basically anything non-toxic and smooth will be fine. Silk plants are preferable to plastic plants, which are often quite sharp, and driftwood, safe rocks, and flower pots can make great caves/hiding places. Live plants are another option, however, I wouldn't rely on them as the sole source of water filtration if you are new to keeping live plants (they have to be growing quite well to remove all the toxins in the water, and it can be a bit tricky to accomplish this at first).
Have fun setting up your new tank!!