That's your choice, not mine.
> none of the ideas I thought about earlier seem that great anymore.
This is very common. I have this when I shower ;) Go stand under the shower long enough and sure, one idea after another. Mostly because I can't access the internet when I'm showering so there is no context. Then as I go and check feasibility, others active in the field, invariably the majority of them pale.
> what to work on ?
Without any stated goals this is a very very hard question.
You'd have to at least come up with a couple of boundary conditions, such as how long you can afford to work on this without any income, how much time you are willing to expend, whether it has to be a strictly one man/woman job, your skill level and so on.
That you quit grad school doesn't mean much, I've seen people with impressive degrees that would not be able to run an icecream stand, let alone a software or service business, and I've seen high school drop-outs that were doing just that and doing fine.
As for small concrete problems, if what you are looking for is work experience instead of running your own start-up for a make-or-break effort not to land in a job somewhere there is a middle ground, it is consulting.
If you're good you could simply hire yourself out by the hour to software companies or companies needing software written for them in your region. If you do that well then roughly 1/4 of your time 'booked' should pay for basic living expenses.
Then you still have 3/4 left to work on you own stuff.
best of luck!