Returnee unsure what to do with the accumulated Zen

I was so excited for STO that I bought the LTS as soon as it was available. Then the game couldn't really engage me and I left quite soon. I've now almost 51k Zen and no idea how to best put it to use. Make it to money (if so, how)? Or buy myself a very expensive ship which I can use from lowbie to forever? As far as I saw I can't buy a ship and sell it in trade network, right? Should I buy lockboxes/keys or whatever the equivalent here is? Convert to dilithium ore? Any input is appreciated!