Keep your electronics and valuables bone dry and organized when it gets wet out.
3L, 6L, 12L, 20L, 30L
A dry sack is another one of those multi-purpose items that is well worth having regardless of your destination. I primarily use it to keep my electronics dry in my bag (you never know when a rain squall will hit or when your water bottle cap isn't screwed on tight), which has certainly saved my gear on more than one occasion. It can also serves as extra storage or even a bag to do laundry in.
The 3L size is adequate to keep electronics and valuables, but if you plan on keeping more than that in it opt for the 6L.
Tip: No matter how waterproof a bag is, moisture is bound to make it inside eventually. To help negate the chances of this moisture from ruining my electronics I keep a few Silica gel packets in both my dry bag and my main bag where I keep my valuables. You likely have some already lying around the house or you can pick them up super cheap on Amazon.