Here are some Apps, which may be useful during your stay in Bonn:

Food

Mensa App:
Offers information on the daily varying menus in all university canteens in Bonn.
Google Play Store Download Mensa App
Apple iTunes Download Mensa App

Transportation

DB Navigator:
The timetable information by Deutsche Bahn includes all public transportation (rail network, tram, subway, buses).
Google Play Store Download DB Navigator
Apple iTunes Download DB Navigator
Website Deutsche Bahn

SWB easy.Go:
SWB easy.GO – the state-of-the-art-App for public transport users in Cologne, Bonn and surroundings!
Google Play Store Download SWB App
Apple iTunes Download SWB App
Website SWB Bonn

FlixBus:
The biggest German provider of intercity coach trips in Germany and to other European cities.
Google Play Store Download FlixBus App
Apple iTunes Download FlixBus App
Website FlixBus

BlaBlaCar:

Offers a platform to connect to or offer lifts by car to German and European cities.
Google Play Store Download BlaBlaCar App
Apple iTunes Download BlaBlaCar App
Website BlaBlaCar

Dictionaries 

Pons Dictionary:
Online Dictionary for many languages. 
Google Play Store Dowload Pons App
Apple iTunes Download Pons App
Website Pons

dict.cc
Offline Dictionary for many languages
Google Play Store Dowload dict.cc App
Apple iTunes Download dict.cc App
Website dict.cc