How can I hang string lights indoors?

Your idea of Command Strips is probably the best way to go. Other ways of hanging the lights will damage the drywall.

www.prettydesigns.com/20-ways-to-decorate-your-home-with-string-lights

https://www.lovelyetc.com/make-super-cheap-hanging-light

Hello Sugar, you can use push pins as they don't leave much of a mark, or

1. Drawing Pins. Probably the default option for many people, drawing pins (we find that brass drawing pins are often best) can be pushed between the twisted wires that often make up fairy lights, as indicated in the image below. ...
2. Blu-Tack. ...
3. Wrapping around household objects. ...
4. Wire Suckers. ...
5. Damage-free Adhesive Hooks.