While I pretty much agree that browsers and such would detract from a dedicated reader, there are a number of games and utilities that can be added to Kobo and Kindle devices. One I like is
Sergey's PBChess package Instalation instructions.
Menu Items, from the applications.ini:
CoolReader = cr3.sh
FileManager = file_manager.sh
OPDS = opds.sh
Dropbox = dropbox.sh
Chess = pbchess.sh
Checkers = checkers.sh
Sokoban = sokoban.sh
Reversi = reversi.sh
Gomoku = gomoku.sh
Sudoku = sudoku.sh
Frotz = frotz.sh
Dictionary = dictionary.sh
Notepad = notepad.sh
Calculator = calculator.sh
Konsole = konsole.sh
WiFiServer = wifi_server.sh
Reboot = reboot.sh
Power off = poweroff.sh
Luck;
Ken