The most important things after tank and filter, in my opinion, would be:
Test kits so you can check your water while the tank is cycling and regularly thereafter. You need ones for ammonia, nitrites, nitrates and ph. The liquid drop kits are far more accurate than the strip kind and are less expensive in the long run because they last longer. If you have Petco/Petsmart near you, go to their website and print the page with the Aquarium Pharmaceuticals Freshwater Master Kit on it and the store will match the online price of $14.99, which has all the tests you need in it and it will last you for ages.
Food for your goldie to eat!
Water conditioner to dechlorinate your tap water and make it safe for your fish. I would recommend Prime-it takes out chlorine and chloramines from your tap water and helps make ammonia, nitrites and nitrates less toxic during cycling. It is super concentrated so it may cost a bit more than other brands but will last you much longer because you need to use so little.
A siphon/gravel vaccuum to remove water during water changes and clean the waste from your gravel, if you are going to use gravel.
A clean bucket that has never had any chemicals or soap in it-for water changes. One with a pouring spout will make life easier.
There are a ton of others things you could get if you like...a stand to put the tank on, air pump with airline tubing and an airstone or bubble wall, gravel (try and get something larger than will fit in your fishes mouth), decorations, plants, hood/top for the tank with or without lights, power strip to plug things in, extra media for your filter, kh and gh testing kits, aquarium salt in case you get a sick fish, a net if you choose to catch them that way...lol, there are a million things!
Good luck with your new fish, and if you have not already, I would recommend reading the following links:
Care tips:
http://www.kokosgoldfish.com/care.htmlhttp://www.kokosgoldfish.com/tensteps.html Cycling the tank (VERY important):
http://www.kokosgoldfish.com/cycle.htmlLOL, vmlola, I was typing this while you posted, sorry for repeating!