I bought one of these (after my 12v battery died without warning):
https://smile.amazon.com/NOCO-GB20-UltraSafe-Lithium-Gasoline/dp/B015TKPT1A/r
I've never been able to actually test it so I'm just hoping that it works when needed. I keep it in the cubby in the hatch area.
Trickle charger is always a great idea for a car that sits more than a few weeks.
For a car that sits outdoors solar panels work well: https://www.amazon.com/ECO-WORTHY-Portable-Battery-Charger-Batteries/dp/B015ZGDYII I attach one of these to my riding mower and it always starts every spring after being in a shed for ~4 months.
Portable jump starters can be handy, I have used mine for a person stranded at the grocery store and a friend's riding lawn mower. This brand has been good for me: https://www.amazon.com/NOCO-GB20-UltraSafe-Lithium-Gasoline/dp/B015TKPT1A
They say up to 4 liter engines and they mean it, mine didn't work on my 5 liter pickup.