can't you just use the shelves for storage of things you already own? It just seems a bit mad to waste money on things you don't especially like, have no connection to, and may not go well together just to fill space?
I'm thinking things like books, glassware or nice plates, perfume bottles, speakers, a laptop, folded blankets, nice bottles of alcohol, etc. (depending on depths of shelves). Or use nice boxes to store non-pretty stuff (just life detrius like random charging cables, matches, batteries, make up, suncream, screwdrivers, cleaning products, blah blah). If you search for 'decorative boxes' you get all sorts, pretty floral patterns, rustic wood, ones designed to look like books or old chests, etc.
Other than that I would find the stuff of yours you do want to display and organise it by colour. No more than two main colours per shelf. Find any nice photos you have that will go with the colours and also any books with matching covers. Hopefully you won't need to buy too much more to add to them and you can start easy with a candle or something in the right colour, or a plain vase with fake/real flower in whilst keeping an eye out for more decorative ornaments.
You could have one shelf just of photos, either use the same, very plain frame for all of them for a bit of cohesion or the same frame but in different colours (e.g. Ikea do a very cheap one in multiple pastels) to create a bit of a gallery wall effect.