we got that heavy duty double sided tape you can get at the hardware store. the letters are definitely not moving and it was nice to avoid drilling holes in the walls (we would have needed roughly 2 holes per letter).
The wall letters we got were from walmart and I painted them the way I wanted.. anyways they had a place on the back for nails. Instead of nails I was able to use thumb tacks (we live in military housing and I didn't want to put huge holes in the walls).
I nailed flat mounting hooks to the back of our letters....PIB (of the letters on the wall, not of the mounting hooks). They hung a bit loosely, so I took them down and rehung them using the double sided mounting tape. That works a lot better and they look really good that way. With the mounting hooks they slid crooked too easily.
I see you have a boy I guess that is why you don't want ribbons What is your nursery theme? This may give you some ideas. If it were nautical you could use rope, if it were jungle then some faux vine would be cool.
Re: how to hang wall letter w/o ribbons...