Some congress-goers see tons of talks, some see none. I wouldn't stress too much about it; do what you feel like, but be sure not to miss out on the non-talk-stuff, because that's an equally big part. For me the "best stuff" is meeting people, and an easy way to do that is to check out the assemblies/projects going on. For more about that see the wiki, which is the main source of info (apart from the Fahrplan) and is always being updated: https://events.ccc.de/congress/2018/wiki/Main_Page

#35c3 on hackint.org is the main IRC channel. Easy way to get answers. @c3infodesk on Twitter is worth following.

This year your wristband is also a public transport ticket valid in the city of Leipzig (zone 110). At night a special tram runs between Congress and the central station (Hauptbahnhof), so no need to worry about getting back.

The alcohol on offer inside is not very exciting, if you care about that. There is however a big supermarket (or hypermarket, rather) nearby, Globus, along with some restaurants/fast food places. No problem bringing in drinks/food from elsewhere to Congress. No security check whatsoever (pointing it out because while this is "obvious" to CCC people it might not be obvious to everyone).

The venue is big: https://35c3.c3nav.de/ is helpful for navigation.

Every year, no matter what I do, I feel I missed out on lots of things and that it ended too quickly. Just explore and enjoy!