Any suggestions for decorating a the shelves and niches?

I love the look of the shelves being painted a jewel tone, and having fabric or wallpaper to line the back. That will show your knickknacks more dramatically. I also like monotone items like black, white, or bright blue. ☺️

Use only your favorite pieces and don't use too many. Find a theme (color, type of item, size) and use only pieces that conform to this theme.

If they're wood you can stain them or paint them different colors. If they are already painted you can strip the paint off or just re-paint. Have fun with it.

I think variety is the key. If you collect a lot of decorative items (which you will, just by gifts or getting things yourself etc) don't think they all have to be out all the time. Some thing, yes, either because they are special or large etc. Here are some different looks on my mantelpiece. I'm posting a lot of pictures. Don't have anyone else's rules but your own. For myself I like doing styling so I change things up a lot. right now we have flowering things in bloom on our small lot and I just had a b.d. so I have lots of flowers on display. My daughter once got to visit Martha Stewarts show back stage. She said they had a very large "closet" with all sorts of things that they can use for display. I like to change things for the season, the occasion, the person etc. It needn't cost much. In the first pic those are bittersweet I picked in the woods and pumpkins. The red hearts I made out of craft paper. The candles I got for free years ago at our town swap shop.

I love the above answers! The key is to less is best as well as the smaller the better! Also, think about sizing....does small to tall work or tall to small as far as the steps? You've got a fabulous neutral color going on which really opens up what you can do!

This is rather odd but true...what story do you want to tell?

for a niche, I would first tape off the wall behind it in maybe a curved arch - with the 2 wall arches meeting about 10-12" above the niche floor if you will. Paint the area under the arches a contrasting color or maybe a brighter white to accent it. take the tape off (frogtape?) and voila! ready for your finest accent pieces.

I think trailing plants would look nice and they would get some light.

This is what I did. Please excuse cluttered table!