Create real acoustic reverberation, even if your interface doesn't have multiple outputs
The essence of real acoustic reverberation is to send a signal to a loudspeaker in a reverberant space, pick up the reverb with a microphone and record it back into your DAW. Let's assume you have a vocal already recorded, and your audio interface only has stereo inputs and outputs. To keep things simple we'll record the reverb in mono, but you can easily do it in stereo with two mics.
Connect one output of your interface to an amplifier and loudspeaker. A guitar combo will give an interesting result, but a hi-fi or studio monitoring amp and speaker will be cleaner. Place the loudspeaker in a reverberant room. (Don't take mains-powered equipment into a bathroom.)
Play the track and make sure the audio is coming through OK.
Now set up a microphone and connect this to one of the inputs of your audio interface. Point the mic away from the loudspeaker so that it picks up mostly reflected sound. Create a new track to record the signal from the microphone. Now comes the important part...
You must mute this new track, so the audio doesn't feed through to the outputs of the interface. You can do this by clicking the mute button or by pulling down the fader. If you don't do this, you will get howlround. This will be unpleasant and you will spoil your recording.
You're all set to go now. Have a run through and set the gain for the microphone, then go ahead and record.
Since you were not able to monitor the reverb, you'll need to play back your recording and see how it sounds. You may find that an adjustment in the mic position will be required. Adjust as necessary and go again.
Hey presto! Real acoustic reverb!