The "box spread":
- European-style options: the pay-off is the risk-free interest rate
- American-style options: the pay-off is a little better due to the risk of assignment
You'll need to offer the MMer a +expectancy > the risk-free rate to see a fill. Conversions are similarly risk-averse, but involve a 3-way instead of the 4-way. It's a very difficult market to game -- there are fully automated systems that execute these w/o any human intervention with direct lines to the exchange.
There are some limited opportunities in hard to borrow short-stock reversals and an occasional mispriced arb due to merger complexities. By and large it's an academic exercise.