My 12-oz. Kleen Kanteen triples as:
1. Hot water bottle
2. Extra water storage
3. Coffee/tea mug
Get the single-walled (not insulated/double-walled) kind and put a beer cozy around it to insulate it.
No plastic taste, no BPAs, no leaching of BPAs and other plastic chemicals into the water (which happens when you put very hot water in plastic), no leakage, and no shrinkage.