I would say some plants and related items, unless you find a fish that you just can't live without. Plants and stuff would probably make your current fish happier, and if you get plants now, and they grow, you can split em apart and begin to decorate the 10 gallon and get it ready for new fish. (or just aquascape a tank for fun.) Right now I am on a failing quest to find small packages of driftwood around me. So, no matter what you decide, good luck. :)
My fish don't have the bare minimum. My two bettas have their tanks finished. I have a very small female in a 2.5 gallon mini bow that is 100% decorated. My male is a small HMPK as well who is only content in his 2.5 that has a large skull (which is loooves hiding in) and live anacharis. They are very much spoiled!
The plants and decor would be for my set up 10 gallon that I have tetras in right now. I wanted it to be 100% live, but I couldn't afford all live plants so I got some fake ones, too. It is fully decorated as well with a terra cotta pot, wendtii, moneywort, java moss, marimo ball, and the 2 fake ones.
So none of them have "bare minimum", I would just be adding on to the 10 gallon set up that is already finished.
I think I'll be able to make $50 from old aquarium stuff I hate or no longer want, and if so, I will be setting up this 10 gallon I got for free and divide and decorate it. I realllllyyyy want another HMPK, so if they really are $8 at PetSmart, I would have a good $40+ to decorate/divide the tank still.
OR, I can buy a 5 gallon hawkeye from WalMart for $30 (w/ tax) and put my current HMPK in there, and use his current decor but buy some more substrate and more live plants...
Then I can sell his mini bow for like $20-25 and hopefully buy a 5 gallon for my girl, too, and sell hers again for around $20, and decorate hers as well?
My two are very happy and healthy right now, so if I don't find a fish that is spectacular, I will try and upgrade the 2 into the 5 gallons. But if I find one that I adore, I will do the first plan! :3