In our home-brew, we don't have spell-slots, but we do have a resource that allows you to cast spells.
Spells cost mana; a 1st level spell costs 1, a 3rd level spell costs 3. There are some things you can do to potentially regain mana during a fight, but generally you have a relatively small amount (say 10). It's up to you whether to cast 3 3rd level spells, or 2 4th level spells - generally, if fights go long you do worry about 'running out of magical mojo' - but with the ability to get some back with a relatively short rest.
For cantrips, as long as you have at least 1 mana, you can cast them and they don't cost anything. Most of them are not particularly useful (if they were, they'd be 1st level spells). Many of the spells that qualify do things like let you speak with animals or creatures of a particular subtype (ie fire creatures). Primarily, they're intended to give players a chance to 'feel magical', even if they aren't particularly practical.
For example, we have elemental schools (like Air) with the following Cantrips: Amplify Sound, Cooling Aura, Speak with Flyers, Ventriloquism. And if you decide to spend time casting Ventriloquism while you're partying at the inn, it won't make you less effective in the fight that happens after you make the wrong person look like they said the wrong thing.