Seriously, go and be bored first, most ideas and inspiration grow out of boredom.
If you can, go for a walk as well, but don't take your phone with you so you get some rest and can relax, this helps alot, plus it is good for one's health.
Just some things i am doing lots when i am out of ideas, it surely helps, upon returning from my walk and head full of ideas, i make a note of every idea and ditch the weirdest/crazy ideas out, just to be realistic in my idea flow.
Out of the ideas that remain, i usually think for a few minutes: "Can i help the world with it or at least some other people?" if no: rethink if it benefits you. if so: go for it!