Resistive heating is, indeed, the simplest. It is also highly efficient. If we ignore the losses in the battery and wiring (which are going to occur in any solution), the power dissipated in the resistive heating element will be converted into heat with an efficiency of 100%.
That's rather difficult to beat, unless a different approach is taken.
The only alternative that I am aware of which can deliver greater efficiency is by way of a heat pump. Common examples of which are refrigeration and air conditioning. A 3.7v Li-ion battery is not going to cut it.
If frequent recharging is your problem, I would suggest a higher capacity battery arrangement - with multiple batteries.