One subplot is the casket plot for Portia's suitors. Portia's father will not allow any man to marry her unless they can correctly choose the casket that contains Portia's portrait. Until Bassanio shows up, all potential suitors have failed at this test.
A second subplot could be the contract that exists between Antonio and Shylock. Antonio needs to borrow some money and approaches Shylock, the moneylender. Shylock loans the money out interest-free, but demands a pound of flesh if Antonio cannot repay the loan.
A part of that plot would be the entire court case. Portia dresses up like a man and is Antonio's defense attorney.
A third subplot would be the multiple relationships and marriages going on: Bassanio and Portia, Nerissa and Gratiano, and Lorenzo and Jessica.
The last subplot is the ring subplot between Portia, Nerissa, Bassanio, and Gratiano.