What looks good, improves a space's acoustics, and isn't too expensive?
echo = reflections of sound-waves from hard surfaces
you have to hinder those reflections and try to absorb sound-waves in your room ...
* https://en.wikipedia.org/wiki/Absorption_%28acoustics%29
if there is nothing in a room even "direct" noise is disturbing - especially if everybody is in a different video-chat and some people tend to speak louder than others.
cheap and good looking?
* hang thick/fluffy carpets onto the walls
* cover other hard/flat surfaces with something fluffy ;)
* put (fluffy) stuff into the room ... from plants to large stuffed animals - whatever floats your boat ;)
* https://www.snoringsource.com/sound-absorbing-materials/
"professional" alternatives
* sound absorbing panels
* acoustic-foam material (some look like egg-boxes, but out of a black foamed synthetic material)
imho this problem - echos and noise in general - is the main reason for avoiding open-plan offices like the plague ... at all costs, these are such an annoyance to everyone!!
especially if you follow a "clean desk" policy, w/o "personalized" spaces and therefore w/o stuff on the desks ... ;))
just my 0.02 €
Edit: forgot the where to put them part of the question which is ultimately unanswerable without know what you are doing and the details of the space including what is in the room.