Answer: I wish it were as easy as just filling in an old swimming pool, but most cities and counties have ordinances covering the requirements for removal and fill-in of a swimming pool. It's a complicated process (removal of all the concrete then back-filling the hole and compacting the soil so it is firm). You can't do it by hand - it takes heavy machinery to compact the soil to the proper degree. I'd suggest checking with your city or county to see what the requirements are.
Even if there were no laws in effect, you really can't just fill the old pool in without providing adequate drainage. Water will collect in the bottom of the pool and it won't take long for it to become stagnant and putrid. (Think of it as a giant cesspool, with no place for the bacteria and odors to go.)
Once you have the pool removed and clean soil hauled in and compacted, you can design your new landscape. You may even be able to find a contractor who will do the removal and then design and install a new landscape while they're onsite.
Best wishes with your project!
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