I recently picked up an A4 sized decor light panel box which came with letters, numbers and symbols and so far its had lots of use. I think using mirrors and light really enhances exploration of light, colour and shadow.
For my toddler (2 year old), introducing numbers.
Looking at colours.
For my 4 year old; letter work.
Looking at insects (grasshopper, wasp and cicada shells).
Looking at beautiful shells (patterns).
Shapes and patterns of leaves.
Painting with watercolour paints on a plexiglass sheet.
I picked up this light box from Aldi in Australia, but they are also available at Kmart Australia. This one was $20 AUD and is battery operated.