You can just create a SKU called “miscellaneous” or “misc”, leave description blank and price zero. When you sell the SKU, the system will ask description and price on-the-fly.
If you do special order or custom work, you can use SPECIALSKU:
You can also create a SKU starts with the word AUTOSKU. When you complete the transaction, the system will automatically create a new SKU use AUTOSKU as a template. This is same as SPECIALSKU, but it will not force you complete the transaction as a special order.