So there must be an easy way of hanging lights inside that doesn’t ruin the paintwork. Or does such a thing not exist?
Apart from ruining the walls, lights fixed with sellotape or bluetack come down at some point before they are supposed to.
When we were kids there were tiny tacks tapped into each upper corner of the room. They were painted the room colour and were practically invisible. They have been up nearly 50 years now
We take down the pictures on the wall and stick tje lights up on the picture hook.
We also have tacks in the top of some shelf units to hold lights
Don't bother?
You can get various shape and size fixings and hooks made by 3M called 'Command Strips' - they have a foamy sticking adhesive pad on and the back and special pull strip. When it's time to take them off you pull the tab and it releases the adhesive from the wall without damage. I'm pretty sure they include little ones for xmas decs
(keep the packet though and pay attention to the instructions for removing them)
edit - actually these ones look like what you need - https://command.3m.co.uk/3M/en_GB/p/d/v000273408/
In the past we have picked up fallen birch branches and set them up to hang lights from, they look pretty cool with the silvery white bark illuminated by the lights. I think we stood them in a disguised tub of sand or maybe a log with a hole drilled in.
These adhesive hooks are transparent and stay up all year. They're practically invisible for the other 11 months.
I think this is their 4th year and they've never fallen down. 
Those Command strips look handy. Might give them a go. 😎 🎄🎄🎄
Those Command strips look handy. Might give them a go. 😎 🎄🎄🎄
Edit: those clear plastic jobbies look OK as well
Command hooks work really well - just follow instructions carefully if/when you remove them.