Make it sound as if there are twice as many instruments as are actually playing:

aecho=0.8:0.88:60:0.4

If delay is very short, then it sound like a (metallic) robot playing music:

aecho=0.8:0.88:6:0.4

A longer delay will sound like an open air concert in the mountains:

aecho=0.8:0.9:1000:0.3

Same as above but with one more mountain:

aecho=0.8:0.9:1000|1800:0.3|0.25