For Apple users on both ends, you can use the Messages app.
Like Signal, Apple's Messages app that exists on iOS and macOS devices uses end-to-end encryption for your messages and attachments. The only problem with using Messages is that its only available on Apple devices, so an Apple user would have to make sure the recipient also uses an Apple device.
WhatsApp.
WhatsApp also has end-to-end encryption, and, like Signal, it can be used on a variety of devices including Apple, Android, and Microsoft devices. WhatsApp also has a desktop version of its app so it can be used on your computer.
Telegram.
Telegram boasts a robust encryption for your messages that you can also set to self-destruct. It's also available on pretty much any device, including Apple and Android for mobile, as well as Window and macOS computers.