Pop-up reminder and todo app?

Sticky notes!

sudo add-apt-repository ppa:umang/indicator-stickynotes
sudo apt-get update
sudo apt-get install indicator-stickynotes

Hope you find it useful.

PS: You can also have: Alarm Clock — Ubuntu Apps Directory

Look at the third picture in the Store and you will see it will also satisfy your needs I hope.

Note: The below mentioned PPA was last updated for 15.10 (Wily Werewolf) and do not have release files for newer releases. Thus, this PPA is unlikely to work on newer systems.

Another way is

sudo add-apt-repository ppa:bhdouglass/indicator-remindor
sudo apt-get update
sudo apt-get install indicator-remindor

working great! See Add and Receive Reminders On Your Desktop in Ubuntu - maketecheasier for more details

There is also the default calendar in XFCE: Orage. It can be installed using:

sudo apt install orage

Once installed, the calendar user-interface is very similar to that found in Microsoft Outlook.