There are two ways you can achieve this now with Planapple:

1. The Itinerary view naturally breaks down your trip by days. So you could add ideas to the itinerary on the day(s) you're thinking about doing them, and then just view them in the itinerary.

Or, if you want to save the itinerary for stuff that's "booked"...

2. Planapple lets you created multiple notebooks within your trip. They're usually used for different destinations, but you can use them for anything you'd like. So you could create a separate notebook for each day in the trip, and then add the ideas for each day to that day's notebook. (The Edit Trip dialog lets you create, rename, and rearrange the "trip sections" which are your notebooks.)