Slots mostly. Maybe there is a certain mod/arcane/warframe that would really ease up your grind and add QoL to your life; and is also a pain to farm (or only available too deep into the game) then you can buy it too.

(I remember back when I started Arcane Energize was an extremely rare and difficult to earn drop; whuch also added a lot of QoL; so I bought a bunch using a little plat.)

As for cosmetics. Maybe a colour pack (most amount of customization per plat spent imo). Maybe a skin for the Warframe you use the most.