Suggestions please!

My friend David arrives in Surabaya (East Java) shortly. We're heading across Java, seeing Bromo (viewing the sunrise from the rim of the outer crater this time, rather than the central volcano), Prambanan temple complex and Borobudur temple (both of which I've never seen) and anything of historical or geographical interest. Possibly will go to land of the Badui, not far from Jakarta, who sound like Hindu Amish.

At some point in Central Java we'll try to see some gamelan (orchestra) and dance. I've heard the dancing at the Yogya palace is very good, so we might aim to be there on a Sunday.

Then it's off to Medan (and possibly Padang) - looking at Lake Toba and some rocks and that kind of thing. All very exciting. Or relaxing, in the case of Lake Toba (apparently Lonely Planet calls it the perfect place to recover from a nervous breakdown).

It seems we'll be in Sumatra during Ramadan - which wouldn't be a big problem in Java, and hopefully won't stop us getting 3 or so square meals a day. We can always tell people we're menstruating. (Menstruating, pregnant women and the sick are not required to fast, in my understanding. Though you're technically supposed to make up the extra days at the end of Ramadan - don't think that applies to pregnant women though.)

Hanging around with locals will be one of our major activities, I expect. And I'd like to hire pushbikes at some point - some of the smaller cities might be nice to see in this way, cycling through the backlanes. I can sample the local unsanitary street food while David watches in horror.

So... I've got a Rough Guide which will give some ideas, but I'm very much open to suggestions. What is a must see, which might not appear in the Rough Guide? Keep in mind that we're both pretty geeky, with an interest in history.