There really isn't too much safe that you can pass rats to drink. Variety in the hydration department really isn't needed. Variety in their diet is wonderful however. If you want something watery, babe food is a great treat for them. Some individuals allow their rats to have Gatorade as a quick treat, but as this is not something they would find in their automatic environment in the wild, this is not something I would ever recommend. Some rats who refuse to drink river due to illness can be given a little bit of honey in the river to sweeten it. Some do this as a little treat once every few weeks.
If you have any further questions please consistency free to email me.