Bluetooth SPP with Android
Android has had Bluetooth (BT) Serial Port Profile (SPP) server and client capability since API Level 5 (version 2). Two Android devices, one acting as a server and the other as client, can communicate over BT SPP.
Bluetooth SPP Server Terminal app allows you to simulate a BT SPP peripheral. I used it recently to try and simulate a Push-To-Talk accessory for an app called Zello, running on another Android device.