How to create a nice secure garden space for a ground floor patio?

You may want to buy an inexpensive outdoor rug...that adds a ton of personality. You can also use a trellis in wood as a partition and can hang flower pot arrangements on it. I bought lots of small planters at the Dollar store & created a mosaic of flower pots...good luck & enjoy the summer!

Barbara: an inexpensive outdoor rug, as suggested by Elaine is a great "space-defining" idea! Getting a wall garden would look terrific!

https://www.amazon.com/SELF-Watering-Vertical-Hangers-Included/dp/B076J79DSV/ref=sr_1_3?keywords=wall+garden&qid=1552654790&s=gateway&sr=8-3

Wow. I never worried about my patio furniture being stolen; I guess that's the days we live in now, though. Keep in mind that if a thief is determined enough, even if you somehow chain your bistro set to the divider wall, they may just cut through the wall if it is made of wood. So consider getting a table and chairs that are foldable and very portable that you can tote inside and slip into a closet or underneath a sofa when you aren't using the patio. For the following ideas, check with management to see if you can do these ideas: (1) Install your own "temporary" fencing by driving posts into the ground and nailing cover to them to give you privacy. While you may not want to go 6 to 8 feet tall with this idea, you can create something of a barrier that is 3 or 4 feet tall by cutting your fencing material (if it's a roll of bamboo fencing, for instance) and zip tie the bamboo to the stakes cut to height and pounded into the ground, or similar idea. You can camouflage the fencing by putting potted plants around the outside of the fencing; you'll still be able to water them and appreciate them because the fencing won't be too tall and if the plants are the kind that grow tall and bushy you'll soon see them peeking above the top of your temporary fencing. If you get snowy/cold winters you can take the fencing down for the season. (2) You might also be able to erect one of those temporary shade structures that people put up on their patios in the summer. You can add sheeting along the sides as curtains to give you privacy that you can open/close. Some of these structures are styled with metal railings to give a feeling of enclosure to the structure. You can anchor the legs by sticking them into painted/decorated buckets of sand to add stability. They might not withstand a bad windstorm, though. So, something to consider. (3) Portable screens that you put up and can anchor so they don't blow over in a breeze and take down and tuck away inside when you're not using the patio. Some ideas here: https://www.apartmenttherapy.com/8-diy-privacy-fences-amp-screens-from-uber-ambitious-to-super-simple-247839

Check what your apartment restrictions are on these things. They can be surprisingly strict and fickle.