The most obvious route would be:

  * Make it more of a reminder service for specific dates
  * Make it email/sms/etc you at pre determined times
  * Make those communications include handy adverts

For example, "I need to buy a present for a thomas who is 8 by Aug 8th at the latest"... on Aug 1st it emails you saying "Don't forget to buy a present! Here's some cool offers, here's a coupon to buy toys. Here's a great place to buy e-cards, etc etc"

I'd start by analyzing what sort of events/items your userbase are adding on your app, and working out how you can enhance their experience using advertising.

If it turned out that 90% of your userbase are adding the todo item "Find a good deal on car insurance", then obviously it'd pay to find some good car insurance affiliate programs... (Hyperbole example, I know, just to show a point).

But 100 users using a todo webapp aren't really that targeted.

By targeted I'm meaning more like "They have already expressed a desire to buy X" etc

The users are targeted, just not explicitly. An advertisement for David Allen's GTD book would target that audience.