A few years ago some tournament sharks had a tendency to take their opponents by surprise by using conventions or conventional systems, against which those opponents could not defend themselves. That is: not without thorough preparation (but how can you do that, knowing you will play only two or three boards against these sharks?).
Quite right therefore that such conventions/systems are not allowed anymore on open tournaments like for instance the very popular (in Netherlands) pub drives.

For those who nonetheless like to spice up their bidding with some 'legal adventure', I recommend for these drives two systems from my youth days.

1 The opponent's system.
My partner and I used to play this in the preliminaries of the Dutch Open Pairs Championship. On arrival at every new table we gracefully accepted the opponent's system cards and started studying these intensively. Of course then there was a short pause, the opponents expecting to receive our system cards. Since nothing happened, they would then ask for them.
With a smile we handed them back their own system cards.
'We play your system, madam.'
Sometimes there was only surprise and silence, sometimes there was laughter (in exceptional cases there was anger, in that case we produced our own system cards; after all the whole thing was meant to be funny).
Of course we frequently and extensively studied their system cards during both the bidding and the play. After all, it is illegal to check one's own card, but not so the opponent's card!
Standard joke: after a bid from my partner I directed a piercing look to an opponent: 'Meaning...?'
We tested the opponent's reflexes as well. For instance after my conventional bid my partner alerted with emphasis.
'What...', usually one of the opponents began, and then, seeing our broad grins, he would join in the laughter.
'You know best yourself', we answered his unfinished question.
This system brought us no points and a lot of fun.

2 System EM.
A very successful system with which we prowled around the tournaments for a while. The system's name was in huge capitals ('EM') on top of our system card. Often nothing happened, but quite a few times we found a victim.
'What does EM stand for?'
'Excuse me?' one of us reacted benevolently.
'What does EM stand for??'
'Excuse me?'
'I did ask what EM stands for.' Slightly irritated.
'Excuse me?'
The record was six times...
Here too: first guess whether it is likely or not your opponents are able to see the joke of it. As mentioned before: the whole thing is meant to be funny…