This is a topic for cool things that a Bionicle Set can do, but was clearly not designed to do. I’ll start.
Four Shadow Leeches can be crammed into '03 Makuta’s chest despite the lack of Tridax Pod.
Rahkshi can wear Kanohi (01-02 style) on their heads. (Just put the little stud in the hole on top of the head.)
Tarduk can hold onto to almost any surface. (This is due to his Onua claws being held they way they are in the “hand” piece.)
Thornax Launchers can fire Zamor Spheres. (Later exploited in Hero Factory.)
Fikou can tuck its head underneath its body. I know it’s a spider, but it gives it a serious turtle vibe.
Vakamas disk launcher can be used as a jetpack.
Disk launchers placed at the back with trans disks create a halo effect on the figure.
Friction adders can be used to extend t and create a neck on sets that have heads very close to their torsos.
Bohrok shields are great as torso armor for metru builds.
Knights Kingdoms limb armor is great as armor for figures.
Round studs are great for filling up unused technic holes.
Lewa Mata already has the strongest attack function of any Toa, but you can make it even stronger by attaching his left hand to the pin hole at the base of his axe. Possibly intentional, though it does tend to knock his mask off.
Pohatu Mata can hold his Koli ball by plugging the studs into the holes of his arms.
Thank you for this. Up until I saw this, my Pohatu Mata was using a light-saber blade to pin the rock to his shoulder.
Back to the topic, Thornax launchers can shoot Thornax into another launcher to be shot again, albeit this only seems to work at close range.
I don’t know if this is intentional or not but Gresh can use his shield as blades and the Piraka can turn their weapons around, also if you wanted you could rebuild Pohatu Nuva to make him look like the other Toa Nuva and move his arms instead of his legs.
All pretty intentional except for Pohatu, that just comes with disasembling the figure
Pohatu Mata can “accidentally” kick himself in the face and knock his own mask off.
You can lock Skopio’s jaws in an open position thanks to the friction between the big Antroz claws and Jaller Mahri’s armor piece.
You know those bags the SWC heads come in? You can leave them in the bags and make spacemen out of your sets.
Nokamas mask when flipped looks like a sea creature that has a balding problem.
Teraks two axels that connect his tail with his function can actually be used to lock into the three axel connector.
Tahu Master can store his shield blade weapons on his back by just using that one axel pin hole connector he comes with.
Tower of Toa and Keetongu include plain dark blue and keetorange bohrok windscreens, these are both part of the colour scheme of fohrok:
You can turn shadow leeches inside out to make lego jello
If you extend Pohatu Master’s boomerangs out all the way, they become these weird bendy swords. Just something I like to use when posing him.
You can store Brutaka’s blades on his back thanks to the gold tubes.