I do enjoy my letter/words games especially on the mobile. Words on Tour is the latest game from Zynga, the game starts in Paris & each level gives you a target you have to achieve to advance.
This target can be simple in play x number of letters to guess the phrase or play a certain block of letters. There are various power ups you can use along the way to help you complete your target & try to three star the levels.
I've nearly completed the Paris stage (currently on stage 27 out of 35) before I travel to London. It is a 'free to play' game but I've felt no need yet to purchase any of the in-app purchases to advance.