Can't put holes in your walls? You can still put up curtain rods.
Home-improvement expert Tom Kraeutler offers alternatives to traditional curtain-rod hardware in The Money Pit, the newsletter he writes with Leslie Segrete of Trading Spaces fame. The two also host a radio show by the same name as their blog
Use special brackets that slip over the corners of the window casings. Brands include Instant-Up Curtain Rod Holders and Ready Hang brackets. The latter have vertical extensions that allow the curtain rod to be hung several inches higher than the window.
If you can put holes in your ceiling, try ceiling-mounted curtain rod brackets from Country Curtains or Swags Galore. For a wall of windows, consider a ceiling-mounted curtain track.
Use the old-fashioned method: a tension rod. It's a spring-loaded rod that fits between the two side jambs of the window. Attractive options are available now, such as the rods sold by Umbra.
Use adhesive hooks to hold a slender curtain rod or dowel. Hooks with 3M’s Command adhesive can be removed without damage or residue, and they're widely available in hardware stores, drugstores, mass-merchandise stores and even some grocery stores.