The basic rule of all erotic play is to use lighter touches when your playmate is less aroused and more intense touching when they are more aroused. Give your partner time to build up some of those fun sexy endorphins. Having your nipples stimulated at the wrong time or in the wrong way can cause pain or even nausea in some people. A great place to start is with a feather-light touch.
Some women or men will find rougher handling of their breasts and nipples, including nipple squeezing and pulling, hard sucking, and rolling of the nipples, to be very arousing.
This is where communication plays a huge part in any play. Demonstrate to your partner how you like to be touched and to give continual feedback about what you like. You can stimulate your own nipples during self-pleasuring or partnered sex. Most partners will find it highly erotic to watch you stimulating yourself.
Beyond the tongue and fingertips, there are nipple clamps ranging in intensity, soft feathers, massage oils & you could try ice cubes on furnace skin or warm liquid on a cooler teat for a nice surprise!