How about everyone list all the wood working tools/techniques they can think of, regardless of how practical they might be as weapons, and then we can look at the ones that will give us the greatest visual spectacle (i.e. bits flying off...) and see how safe/unsafe they are...

Drill & Drill Bit
Jigsaw & Blades
Circular Saw & Blades
Chainsaw
Miller & Cutting Heads
Band saw
Hammer & Chisel
Sander (All types)
Steam Bending
Router (Miller Variant?)

Another possibility... we put a weight limit on the amount of metal allowed. Say 5% of the robot max, 680g. You still say no metal armour but then people can use that weight to fit metal bearings, shafts, a pulley and a disk. By the time you have got the mountings done you might only have 400g for the disc itself. If you ban titanium and case hardening of materials (before or after purchase; so Hardox is out) then you can limit the amount people can/want to spend as well.