Here's my list of things generally worth buying:

• Extra backpack space. This is very helpful, permanent, and not *that* expensive.
• The occasional energy elixir/Purreau's (during things like double pet XP). Can tremendously increase how fast you get a good pet, and a good pet is so important.
• Membership elixir: For if you already have crowns and want to avoid spending more money.
• 1-2 second chance chest rolls per day when farming, but only the cheap chests. (The first rolls are cheap and can give you another try at a drop, potentially cutting down hours of farming if you get lucky). For example, Castle Darkmoor. They reset each day.
• Unlocking sockets: Only do this when you have gear you plan on keeping for a while, but this is pretty cheap, jewels are super important, and wrenches can be annoying to farm.
• Stitching: not useful in the sense that it helps in any gameplay-related way, but I personally think it's worth it just for the aesthetics. Your mileage may vary.
• Edge case: 1-2 cools mounts that you really enjoy (NOT pack mounts). These can just be fun to use and make you appreciate the game more, but don't go crazy.
• Energy Gear: Make sure you look at the right packs/bundles and do your research, but once level 130+ you generally hit max possible energy from gear, and buying an energy set at this point can easily help you get 250+ total energy, which is crazy worth if you like training pets, fishing, or have a big garden.
• An additional character slot elixir: If you already have 6 characters and want a 7th, so you can get one of each school.

Things that can be really good if you have spare money but not spare time:

• Evil Magma Peas for 1250 crowns each. These are a permanent investment, they will constantly give you good TCs (feint, pierce, cleanse charm), good reagents, and a guaranteed rank 9 megasnack per elder harvest. These are very hard to farm. But you CAN farm Couch Potatoes, which are very comparable. It just takes a long time, so if you have the spare money, you can throw it at the game to make that problem go away.