I bought these domains. What can I do with them?

I think you have the app creation process in reverse...rather than starting with the domain and brainstorming ideas, you should look for ideas that solve a problem and come up with the domain later. There seem to be lots of reminder services and apps out there, and every bank and credit union already has bill pay. People usually know things are due...the real problem is finding the money to pay bills or the time to complete whatever is due. Maybe you could solve that problem with your app.

Its true that one needs to build a product first then worry about the marketing. I just happen to grab these two domains so I kinda skipped the product part first.

Thanks for your suggestions/comments.

Aside from bills, maybe also consider some kind of personal renting app? For example, if my neighbor wants to borrow my lawnmower, your app could help set up the due date, automate the reminders, and maybe even take care of compensation?

Along the same train of thought, if someone owes you money, maybe have this handle the amount, terms, and reminders for the borrower (small amounts)?

Personally, I'm not the type of person to hound someone for either of the aforementioned items. I think that'd be a neat service.

Domain names are about 3% of the "product". You could just as easily make IsDue.by the premier place to buy and sell used parts for vintage Snapper lawnmowers, provided that there was a market for such a service.

Similarly if you invented groupon you could have probably also used the domain fuckmeintheassandrapemydog.com and you'd STILL have midwest housewives DDoSing your servers everyday.

A domain without an idea is just a random collection of letters.