Brainstorm...
If all you're looking for is to burn cash, the quickest and simplest thing I can think of would be to buy a bunch of fighter jets (lawn ornament) and keep them in your family inventory.
If any of your sims has the Good trait, he/she can donate to charity over and over again, but that would take quite a while.
Buy a ton of laptop computers and leave them in as many public places as possible.
Go to build mode and continuously resurface the largest floor/wall space in your house, back and forth between two textures.
Do you really like building/rebuilding? If so, cater to your inner architect/pyromaniac. Place lots of fireplaces in your home and turn them all on, unattended, while your sims are out. Delete all exterior doors. By the time the blaze goes out (and yes, there will be a blaze) there will be lots and lots of destroyed stuff that insurance won't fully cover replacement costs for.
Buy large quantities of the most expensive items at the grocery store. Throw them all in the garbage.
If all these methods combined somehow cannot keep up with your income, don't be too worried. As revealed in another thread here, the game limits your household net worth to about 2.1 BILLION SIMOLEONS, so you're nowhere near the playable limit!