Hellobee Boards

@Foodnerd81: Lowe's and Home Depot sell fencing that you stake into the ground. You can attach a gate to it also. We used it around our pond at our last house, and it is easy to install and pull up later:

https://www.lowes.com/pd/Ironcraft-Common-4-5-ft-x-6-ft-Actual-4-5-ft-x-5-8-ft-Berkshire-Black-Powder-Coated-Aluminum-Decorative-Fence-Panel/3469017

Along the same lines as @ladybee, we have these tough boxes (our Costco sells them for $6.99 each). They are great to store off season stuff (which will weigh them down).

Homz Durabilt Tough Storage Tote Box, 27 Gallon, Black With Yellow Lid, Stackable, 4-Pack https://www.amazon.com/dp/B015DZJNOA/ref=cm_sw_r_cp_api_13o-ybAV9PKAH

We used these to make a makeshift play kitchen for DS on our patio. It's essentially counter height for him. We put a bunch of play food and utensils out there. The lids are concave so you can fill one with play sand and make it a sandbox she could stand at too. A covered grill might also be able to block off part of it, if you have one laying around.

If you're really just looking for quick and dirty fencing, I would get safety netting - it's that plastic chicken wire looking netting they use for temporary fencing on construction sites or plastic garden netting (to keep deer away). They sell it by the roll at Home Depot for like $30.

It looks like you can zip tie it on either end on the deck fence and the gutter pipe, but you would need a stake or pole at the corner. If you can't post one in the ground, I'd find something tall and heavy the netting could wrap around, like a weighted garbage can or a grill placed on the corner of the deck. If you go this route you can use the same stuff at the top of your deck stairs I guess - you would just have to replace it every time you need to use the stairs, but the rolls of netting are long and Zip ties are cheap!