Yes, the good thing about towels is that they are free short-term solution to test things out. Sorry about that CKoffend as I forgot to mention that.
There are many products out there and some of them are indeed quite costly. Actually, you can try out Auralex products as they are quite reasonably priced. There is another product you could consider -I normally don't recommend this for long-term use as most folks do not endorse it, but if you want to test things out the cheapest way, try
this either the 2"-3" wedges or pyramids. Ugly on its own but much more effective in absoprtion levels. For short-term testing only, much more effective than towels, not free but the cheapest.
For aesthetic reasons, these can be framed up and covered with acoustically-transparent fabric, but then again it would cost quite a bit.