I plan to visit Europe for the first time in March and since the cheapest tickets (non-direct) usually involve a layover in Iceland, I decided to spend a week in Iceland, before continuing to mainland Europe (Spain and beyond). I'm still in the process of figuring out my itinerary within mainland Europe, but I plan to stay in Schengen Area for about 2-2½ months, then take a train to the UK, and then finally return home.
I know that as an American I can enter the Schengen Area visa-free for 90 days, every six months. I have considerable savings (25K+ USD) to support my stay/travel but, since I travel alone, I plan to stay mostly (but not exclusively) in hostels, CouchSurf, Airbnb etc, around Europe to meet people. I'm trying to keep my itinerary flexible.
How much proof of my itinerary/lodgings/etc do I need to show and do I need to have proof of an onward train ticket (say Paris-London via Eurostar) when first entering Schengen in Iceland?