Is it furnished or unfurnished? If you are putting your own furniture in there then its just like normal.
Otherwise, nice throws on the sofa. Nice lamps, cushions, rugs. Large pot plants.
If you have any paintings/prints get them up on the walls. I know some agreements say you can't put anything up on the wall but IMO if you don't go crazy and limit it to two nail hoes per room that is easy to fill and paint over when you leave.
You can always ask to paint the odd wall, but permission isnt always given. Despite stating neutral only, Ive had walls painted red GLOSS (yes really), skirtings painted black, red and black stripes, paint tipped over carpets and just really bad painting...
Now, where I have given permission Ive always asked that when the tenancy ends the walls are returned to the original colour. I havent needed to insist on this as actually so far theyve all been done really nicely, but it does mean if I ever had red gloss again I could get them to repaint or take it from their deposit.
I think with this sort of request its always best to have a good (honest) relationship with the ll, perhaps invite them over and show them the paint colours.
Rugs, throws, pictures (second the command strips/hooks), ornaments, books etc. Just unpacking now, and it's beginning to look like home. Will be repeating in 2.5 years when I finally get to move home, but back to my house, so can do what I want with it. Have also put up my own curtains, which helps.