Honestly, picking your first lens for a big trip is stressful, but you made a killer choice with the S5II. Ive been shooting with that body since it launched and tried many different zooms for the L-mount system. If you have 800 left, you want something versatile but light because youll be walking miles in Italy and heavy glass is honestly a nightmare. In my experience, the best all-rounder that fits your budget is the Sigma 28-70mm f/2.8 DG DN Contemporary. I have used this lens for a few travel gigs and it is tiny compared to most f/2.8 zooms out there.

• Constant f/2.8 aperture is a lifesaver for dark cathedrals or evening dinners.
• Weighs almost nothing so your neck wont be killing you by day three.
• Fast autofocus that actually keeps up with the S5II phase detect. If you feel like you need more width for the narrow streets in Rome or Florence, you might consider the Panasonic LUMIX S 20-60mm f/3.5-5.6. It is way cheaper, usually around 400 or 500, which leaves you plenty of extra cash for wine. Over the years I have learned that doing everything usually means choosing between low light speed or zoom range. The Sigma gives you that professional blurry background look, while the Panasonic 20-60mm lets you cram more of a building into the frame. I would personally grab the Sigma... that f/2.8 is just too good to pass up when you are shooting indoors or at night.