My friend and I were looking at painting my other friend was placing down. we then realised if we looked through each other's translucent player heads that appear in spectator mode the painting and our friend (including their armour, shield and item he was holding) would disappear. We then experimented some more and found out if you were to look through your own head in f5 then it would happen the same. We then experimented for many different things and this was the list of things that we found:

Players (and items they are holding and wearing)

Paintings,

Item frames,

Signs,

Beds,

Dropped items,

Chests,

Ender chests,

White stained glass panes and blocks (it doesn't affect normal glass),

Bells (not the wooden part though)

Nether portal gateways,

Mobs were not always showing but it would require more investigating.

These were the only ones we investigated but I am sure that there are many more out there.