How can I best decorate these windows in my powder room?

To be honest I would remove the curtains and use a privacy film for both that is available in different patterns

Check out Pinterest, they have so many ideas for different and unique ways to dress windows, here is one link:

https://www.pinterest.com/explore/unique-window-treatments/?lp=true

I'd leave the one over the sink open to show the detail and put frosted contact paper on the other. You could then use the sill for a plant, décor, etc.

I too would remove the window treatments and use privacy film. It would put the focus on the light and the rest of the decor.

I would be tempted to hang an oval MIRROR over the window facing the front of the house. The 2nd window has such a deep sill, you might try tension rod INSIDE the window and hang shells strung on strings from it ... although I do like the Privacy film. There are some nice patterns of it available on wish.com.

Take down the curtains!!!!!!! Your round window is so unique and you should let it be seen. Take down what I think is a blue solo cup. Cover the glass with a privacy film and do not put anything on the sill. Do this for the other window as well.

Buy some beautiful towels. Fold over top of towels, this will form your valance. Leave enough room at top of fold for your rod to fit through. Sew straight across. And u have it, curtians.

I too would lose those curtains, and put decorative window film in.... here is one for your beachy theme... https://www.amazon.com/Lighthouse-Etched-Window-Decal-Vinyl/dp/B004I9YQ18/ref=sr_1_3?ie=UTF8&qid=1502493974&sr=8-3&keywords=decorative+non-adhesive+window+film+and+beach

You could put a piece of frosted film on the outside of the oval window or use the frost spray.

I might put a small glass shelf below the oval window above the sink, and put some pretty sea glass accessories. https://www.amazon.com/s/ref=nb_sb_ss_i_1_16?url=search-alias%3Dgarden&field-keywords=sea+glass+bathroom+accessories&sprefix=sea+glass+bathro%2Cgarden%2C332&crid=3JE88GOIPP8U4 you will note there are some collections of the painted mason jars in sea blue for the bathroom.

Here is a cute little shelf tower that might fit next to the sink you could put towels, soaps, sea shells on.... https://www.amazon.com/InterDesign-Standing-Bathroom-Storage-Accessories/dp/B003YUBNTA/ref=pd_sim_196_3?_encoding=UTF8&pd_rd_i=B003YUBNTA&pd_rd_r=T68ZM8SXEQWQJ3V3NX8J&pd_rd_w=16fCz&pd_rd_wg=ibqMV&psc=1&refRID=T68ZM8SXEQWQJ3V3NX8J

Lastly, I would paint the walls a more vibrant sea blue, aqua, or sea foam green above the chair rail and even darker blue around the window frame.... https://www.google.com/search?q=sea+glass+paint+colors&client=opera&hs=SR1&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwi2nvvarNDVAhUQ0GMKHat3C6MQ7AkIOA&biw=1366&bih=629

Take down the curtains & also the picture above oval window. Yes use film over oval window. Leave the other window plain with small flowers. I wish I had the oval window, loos fantastic. Good luck!

Mare

Use glass paint and create a unique design.

I would get a very large mirror and just cover the oval one up and large enough to use above your sink. Could glue rope around edge and glue few shells on .The square one, buy beach fabric for curtain. please think about taking some of the pictures down ,to many for small room, if you don't mind me saying.

I would accent that round window by adding rope around it and beach film and omit that curtain. On the other window insert a white mini blind so you can choose the light flow and add a boat oar above that window and place hooks in it for using it as a curtain rod the ones with tab top or grommets and get a sheer curtain and pull it to one side and add a rope tie back and glue a star fish to it. Add wire waste paper basket and glue rope to it. I have photos to help with the project.

I like that last pic of the oval window" its a decor statement"😀great idea