If you want to see non-Meraki devices on the network, you can use...
https://developer.cisco.com/meraki/api-v1/get-network-clients/
This returns some info on client devices, the amount of detail will vary with device manufacturer, type etc.
{
"id": "k74272e",
"mac": "22:33:44:55:66:77",
"ip": "1.2.3.4",
"ip6": "2001:db8:3c4d:15::1",
"description": "Miles's phone",
"firstSeen": 1518365681,
"lastSeen": 1526087474,
"manufacturer": "Apple",
"os": "iOS",
"user": "milesmeraki",
"vlan": "100",
"ssid": "My SSID",
"switchport": "My switch port",
"wirelessCapabilities": "802.11b - 2.4 GHz",
"smInstalled": true,
"recentDeviceMac": "22:33:44:55:66:77",
"status": "Online",
"usage": { "sent": 138, "recv": 61 },
"namedVlan": "My VLAN",
"adaptivePolicyGroup": "2: Infrastructure",
"deviceTypePrediction": "iPhone SE, iOS9.3.5",
"recentDeviceSerial": "00:11:22:33:44:55",
"recentDeviceName": "Q234-ABCD-5678",
"recentDeviceConnection": "Wired",
"notes": "My AP's note",
"ip6Local": "fe80:0:0:0:1430:aac1:6826:75ab",
"groupPolicy8021x": "Student_Access",
"pskGroup": "Group 1"
}