I have a few ideas
Central Cavern - do the jump from the conveyor to the higher platform first time (requires pixel perfect jump on a conveyor so quite tricky) then get to the exit without dying. Secret level warp to ZX81 version of Central Cavern (which is harder I think)
Cold Room - do the chimney jump to enter the exit from above rather than from the side. Secret: warp to Cold Store from JSW but Manic Miner style (I'd put the exit on the cliff to the right requiring a rope jump to get to it). Requires me to implement ropes though. Easy to get this achievement of course.
Processing Plant - make the jump to the exit at running speed (i.e. no lining yourself up), first attempt (that's another pixel perfect jump). Reward: Pac Man style cutscene with Willy chasing the pac man guardian.
Both Amoebatron levels: Never land on the platforms you don't need to step on cos you can make the jump from lower platform to middle platforms without stepping on them. That's not difficult especially if you can line yourself up. Reward: Dunno
The one that is puzzling me is whether it is actually possible on Eugene's Lair to collect the key on the right hand side last (instead of the obvious way of collecting key nearest to the exit)... every time I attempt it I fail by it looks like a single pixel lol. Is that actually doable? It would surly require Eugene to be right at the top or very near it on his path. I think it might be doable but I'm not good enough at MM lol
![Wink ;)](./images/smilies/icon_e_wink.gif)
Also looking for more ideas for challenges (they don't all have to be really difficult of course, some could be only mildly challenging).
EDIT: So looking for unusual key collecting order/hard to make jumps etc. that makes the level significantly more difficult etc.