Nice to see you getting out. 😉
<blockquote>How much money should I bring along? Cash, traveler's checks?
</blockquote>
Enough to get to Heidelberg (by train) and get out of trouble if you need too (50-100 Euro cash should do it). You can buy your train tickets with credit card in Germany.
<blockquote>How plentiful are ATMs?
</blockquote>
All over the place (but know the pin numbers: the order of the alphabet on the numeric key pad is different or missing all together -> friend of mine who spelled his PIN out using the alphabet and caused his card to be blocked by messing up more than twice in a row)
<blockquote>I have already verified with my bank that my ATM card will work, and I have the big three credit cards as well.
</blockquote>
You should be fine. Plentiful ATMs make traveler's checks more of a hassle than a help.
<blockquote>What sort of weather should we be prepared for?
</blockquote>
Spring weather. Sunny, possible spring rain shower, cool, not cold.
<blockquote>We hear that "everyone in Europe speaks English these days" but I suppose that's not universally true. If I need to ask directions, find a bathroom :), etc, how likely am I to be able to do that in English?
</blockquote>
Very likely (also: English is just a German dialect anyway so you should be fine).
<blockquote>Any other travel wisdom to share?
</blockquote>
Take as little as possible with you and do not panic, things tend to work out here too.
