easy - get toon slots, create KDF toons, EDIT: PARK THEM AT DS9 RIGHT AFTER GETTING THE FIRST DOFF PACK, (you'll have to do 2 missions with every toon) collect contraband, convert to dill, convert to zen
At my stage in the game (I'm a Vice Admiral with two characters; one FED the other KDF) I would spend your Zen on Fleet Ship Modules.
The majority of my Zen (past-tense) has gone on Bank and Inventory slots (though I've still a few to unlock if I wish), a couple of master keys (more than I'd like to admit) and retrain tokens. I've never purchased a uniform (as I feel the stock ones do justice) and I've never felt the need to buy a zen ship when they're all outdated to Fleet ships.
Playable races. Lockbox keys and Fleet doff packs for selling on the exchange. Two more character slots... Not four. Just two. You'll be spending a lot of time babysitting even your DS9 parked KDF contraband farmer that would be better spent shooting things.
EC cap increase. Account bank. VA level ships. TOS/ TNG/ DS9 costume packs.
Everything else. Especially anything bound to a single character and not account wide. Like the Borg BOff unless the plan is to sell it for big EC.
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Slots are what i buy when there is nothing left in the C-Store that i want... with 11 Characters there was a lot of Slots to buy, that said, i only have a few bank slots left to unlock...
i guess after that i'll go for the last 5 or so c-store ships, just to have 'em all... and then i either keep my Dilithium or go for LockBox stuff again... i don't know.