The Marias- and the Place de Vosges in particular- is my favorite place to get lost in Paris. Due to the sporadic curves or lengths of the small streets in this neighborhood, you need to just let the city take you in and know that you'll be wowed by every new corner you see.
After you've spent some time lounging on the grass at the Place de Vosges, possibly napping to the sound of the falling fountain water, head towards the southwestern corner of the interior of the Place de Vosges and look for the open wooden door (and pathway) that leads to another open courtyard. You'll see signs for the national office for French monuments and know that you're on the right track. One more walkway on your far right and you'll wind up in the enchanting 17th century garden of Hotel Sully that is certain to sweep you off your feet.