If you do not want to use a diode, and your driver transistor can withstand it, you can use a MOV across either the transistor or the relay coil. Rating for the MOV is selected by using a MOV with a DC voltage withstand rating that is just a little above your supply voltage, and with a max clamping voltage that is below the transistor voltage rating ( or the suplupply plus the max rating if across the relay coil), which has the advantage of being faster in turn off, as the coil can generate a higher back EMF and dissipate the stored energy faster.