Tantrums are a tough one! I try to stay calm and use distraction techniques - it's so hard when they're little and you just want to pick them up and cuddle it all away, but of course, that doesn't always work as they get older!
I've found that giving two choices has helped; often the simple 'would you like X or Y for lunch?' helps me navigate the tricky territory of offering something healthy that they will actually eat vs. full-blown meltdown because it's not their favourite food. Giving them some autonomy seems to help, even if it's just an illusion of choice!
boundaries are harder - I'm interested in hearing other people's strategies too, as mine tend to vary depending on the situation! Timeouts don't really work for us, but taking things away seems to hit home sometimes - especially with toys or privileges like screen time. But then I feel mean!! Argh, parenting is tough!