What are some simple ways to reduce Arduino power usage?
My first step would be to identify what is using most of the power/current and address that.

I often see these questions about reducing power consumption / increasing battery life on this site they often mention the general solution you already list and which are listed in other answers.

For example I agree that reducing the supply voltage of a microcontroller reduces power consumption. However, if the uC is mostly in sleep mode and only active 1% of the time then reducing the consumed power is only of relevance if the uC takes a significant (for example more than 20%) of the total power budget.

If for example your temperature sensors are on continuously at 1 mA each that's 2 mA total at 100%. Compare that to a uC being active 1% of the time at 10 mA gives an average of 1% * 10 mA = 0.1 mA so 20 times less.

So the conclusion there would be to duty cycle the temperature sensors. Make the uC switch them on/off (or their supply, perhaps you can simply supply the Vdd of the temp. sensors from an I/O pin on the uC). Even if the temp sensors are only stable after having a supply voltage for 5 seconds that would still help significantly if you do a temperature measurement once per minute.

I usually make a table with time active (in %, so basically that is the duty cycle), current consumption and the effective average current (which is simply the product of those two).
That helps me identify where the current/power is going and that tells me how I can improve it.

Concerning the step-up converter: you might not need it if all components can also run on 3.5 - 4.2 V. The ATmega chip can, some can even work at 1.8 V (you might have to change the "Brown Out" voltage in the fuse settings though). Some step up converters have a low quiescent current (current drain when the current at 5 V is zero) but not all do.

Most circuit designers like to have a stable supply voltage, say 3.3 V. However, most chips actually don't care! As long as it is in their usable range. For high accuracy / low noise things might be different of course. My point: you don't always need a stable/regulated supply voltage. Removing that LDO / step up converter can save a bit of current.