river rock

decking... use it like a table.

Hope this article can help you https://www.quictents.com/blogs/news/backyard-landscaping-tips-for-dog-owner

Pavers would be easy to walk on and stay in place. Les expensive would be bigger rocks with sharp not rounded edges that would be more likely to ”lock” in place than pea gravel which is easily dislodged.

There are several options you could consider for the area between your retaining walls:

1. Artificial grass: This could be a low-maintenance solution that would resist digging by your dogs and wouldn't require water or maintenance.
2. Poured concrete: Pouring concrete in the area could create a durable and easy-to-clean surface that your dogs wouldn't be able to disturb.
3. Interlocking pavers: Similar to concrete, interlocking pavers can provide a durable and low-maintenance surface that would resist digging by your dogs.
4. Bark mulch or wood chips: Bark mulch or wood chips could provide a softer surface for your dogs to walk on and would break down over time to improve soil health.
5. Large stones: Placing large stones in the area could provide a natural look while also resisting digging by your dogs.

It's important to consider the size, activity level, and habits of your dogs when choosing a surface for the area between your retaining walls. You may also want to consider the local climate and any drainage requirements for the area.

I’d put shrubs, ornamental trees or even low maintenance grasses in massive pots, pots tall enough that the dogs can’t pee on the foliage. If the budget allows, add several. Ask at a good garden centre (not a big box store) for plant recommendations if need be. You don’t mention your gardening zone or area.

A low ground cover. Something too short for a dog to lift a leg to pee on.

he's marked it already so if you do put something there, everything should be dug out, and surrounding area changed out too. use a good wash down to get the smell off the hardscape.

replace that corregated material w/something taller and heavier. you see how this taller wall area is done against the grass? something like that

with the higher wall behind it, and larger rocks and prickley shrubs, the dog will change his mind

another option, do away w/the planters and bring out the grass level

this looks pretty. train the dog to go elsewhere

you could use larger, sharper rocks for the cover, and bring decorative boulders to set on them.

I like the idea of the last picture that bette posted

I know you said you don't want to create a garden around the stairs, but I couldn't resist the temptation to create an inviting space. Maybe you'll change your mind :)

Pea gravel and loose mulch are both poor choices if you have wild and crazy dogs and kids. If you want to use rocks, you need much bigger ones, at least cobble size. Probably bigger than cobbles.