Çinili Camii (Tiled Mosque)

Lonely Planet Istanbul makes this small mosque in Üsküdar/Tabaklar on Istanbul's Asian side sound like a real hidden gem, saying "[it's] unprepossessing from the outside, but boy oh boy - wait till you see the interior".  I think that's more than a bit of an exaggeration; unless you've never seen more than two Iznik tiles side by side (and if you've been to the Blue Mosque, you definitely have, and to a much grander effect), you're probably not going to be blown away.  The inside is quaint and nicely decorated with tiles, but not exactly what I would call spectacular. 

The friendly caretaker opened the mosque for me and showed me around.  Although he didn't speak English (but knew a bit of German), he was very patient with my almost total lack of Turkish and politely asked for a donation for the mosque as I was leaving (I gave 5 YTL, which seemed perfectly acceptable to him).

Even if the mosque itself isn't a must-see, however, I'd still recommend it, since the 20-30 minute walk up from the Üsküdar ferry dock will take you through an everyday, working-class Istanbul neighborhood, at least in comparison to Sultanahmet, Beyoğlu, Taksim & Nişantaşı.  It's not easy to find, but if you get lost just ask someone and they should be able to at least point you in the right direction.  The Atik Valide Camii, another of architect Mimar Sinan's masterpieces and apparently quite significant in terms of size and architecture, is also nearby and along the way from Üsküdar, although the interior was closed when I stopped by and no caretaker was to be found.