The only thing that needs a bit of planning ahead (about a month) is visiting the Bundestag - fast way to discover it is to just go up to it's dome. You can book a table at the restaurant up there if you want to have breakfast above Berlin. In addition the visitor service offers various presentations for free that can be booked from their web page.
Some hints in addition to visiting a tourist information after your arrival:
When I have guests I usually recommend to either buy a day (or week) BVG ticket - you can use public transport as often as you like with these tickets. That includes S-Bahn, tram, busses, tube and ferries (but not the tourist roundtrip boats with moderation). If you know you'll be going to several museums, a Welcome ticket might be worth it's prize. Alternatively just get a bike - unless you want to reach destinations outside the s-bahn-ring or want to visit in winter (don't) all distances should be easy to do by bike. To plan your trips use bbbike.de - they know road conditions to e.g. let you exclude larger streets or prefer green routes.
Your best bet to see most of the attractions for less than five Euro is to take the regular bus line 100 from the Bhf. Zoo train station down to Alexanderplatz and line 200 back. Though no audio guide is known to me there should be guides available for sale in local tourist information offices.
For guide books: Lonely Planet is a good start. If you speak German the city box might serve you well. It contains 30 cards with proposed walking tours including brief explanations. Also the book "Die schönsten Berliner Stadtspaziergänge" has been great to discover areas that are less known.
The city has two bi-weekly magazines that feature lists of concerts (both modern and classical), exhibitions, markets and more: For one there is Zitty, the other one Tip Berlin. Both are quite good, which one to prefer depends on personal taste. In addition both publish restaurant guides, books on where to go shopping, special issues on where to go and what to do. In addition their online restaurant reviews are quite decent.
Two final hints: If you happen to know locals (or anyone who moved their a while ago) - make sure to ask them for recommendations. Also, try to stay at one of the many B&B locations - in general you host will know several local recommendations.