The area directly outside Inkopolis Tower is called Booyah Base. It is where all the shops are located. The shops are Shrimp Kicks, the shoe store; Jelly Fresh, the clothing store; Cooler Heads, the hat shop; and Ammo Knights, the weapon shop.
Each store can be bought from with the in- game currency, which is earned by playing Turf War and Ranked Battle. These can be accessed from Inkopolis Tower.
There are also a few other things to do in Inkopolis Plaza. First, there are many Inklings walking around the plaza. You can check what kind of gear they have to order it. It can then be picked up from Spyke, who is a sea urchin that you can pick up gear from or enhance your gear with Super Sea Snails.
The second thing is the single-player campaign. Cap'n Cuttlefish will peek put of a manhole. Jumping into this manhole will allow you to access the single- player campaign.
The third thing is amiibo challenges. By using one of the Splatoon amiibo, the player can unlock challenges. Once all 20 challenges are cleared, the player will unlock a special arcade game that they will be able to play while waiting for a match or from an arcade machine in Inkopolis Plaza.