Look the bike over and see how many wrenches it takes to deal with each fastener. It might not be too many. On my mountain bike a single folding hex tool covers all of them. I then add a tube, tire levers, patch kit, pump, chain tool.

I realize a cruiser probably has a more diverse set of nuts, bolts and screws, but see what it would take to cover all of them. Then filter out the ones which are unlikely to cause problems. On an old cruiser you might just replace the hex tool with a box wrench or adjustable wrench.

The most likely problem will be a flat tire. A patch kit will get you going, unless the problem is near the valve. It is a real pain to patch on the road however, and sometimes the patch needs a little time to 'take'. A tube replacement requires the wheel to come off, so if you bring a tube, also bring a wrench for the wheels unless you have quick release.

Another common issue is the need/desire to alter the seat height. Nice to be able to do that.

A broken chain will end your ride unless you have a chain tool to remove the broken link, but if the bike is set up such that you cannot make it work with a shorter chain, then there is no point in bringing the chain tool, unless you also bring extra chain links. A bent chain link stranded me recently on my shopping bike, but the chain tool saved my day on at least one other occasion.

I like to make sure I can handle a flat, and have the ability to tighten things which might rattle loose like handlebars, seat, cranks, bottle holder, rack, etc.

Or leave all of it at home and carry a phone.