Yup some of our favorite open ended materials are:

Scarves in silk, satin, gossamer and other fun glitter shiny sensations and various pieces of fabric in different textures, patterns and colors ... these get used for creative movement, for 'dress up' and fort decoration and sometimes just for snuggling up with while they read a book.

Sea shells, river rocks, pine cones, stick collection ... these make awesome 'patterning and mosaic' for play and get used for food in dramatic play and other props in role play.

Gemstone collection in a lazy Susan tray ... those glass gems meant for vases the kids love to sit at the table and create designs and pictures with the stones

The 'electronics' bin .... various old cameras, cellphones, cordless phones, binoculars for adding to play

The paper clip bin ... various sizes and styles of paper clips and I created a 'grid' for them over the opening in the bin with gimp to hang and weave with the clips

The marble bin .... 2000 marbles in an under the bed storage container (long and deep) with only METAL implements to explore with - tongs, spoons, silver tea tray and cups ... noisy as hell but they LOVE that bin one because it is noisy and two because you have to be 'big enough' to earn the right to play with it so it is that right of passage to get to play there

Weaving and lacing bin - have one for two age groups mouthers and non mouthers ... so the mouthers get pipe cleaners and larger beads, buttons and things that will go on the pipe cleaners and the non mouthers have a similar bin but with smaller 'treasures' and lengths of gimp and ribbon and so forth.

Also have collections of pom poms, buttons, poker chips, bingo chips, play doh lids, marker lids from dried up markers and other well 'bits and bob' type treasures that they can explore either on their own sorting , on the light table, use in creative art or 3D creations and so forth.

To be honest MOST of my materials and collections are things from 'real life' collected at garage sales, second hand stores and from clients .... and these are played with far more than 'toys' from Toys R Us or Walmart!