I'm planning on finally ordering our German rail pass before the sale ends on this site. BUT, I'm forever a worrier, and I'm so afraid that right after I order the pass, I will realize that I should have bought more days than what I purchase. Once I buy the pass, I can't purchase extra days, can I?
I don't think so. But if you are making only shorter trips, you can usually pick up a daypass - either a Laender Ticket or a Happy Weekend Ticket - nearly unlimited travel on the regional trains. A single version can cost under 20 euros; a pass for up to 5 people traveling together won't cost you more than 33 euros.
Read more here:
www.bahn.de/p/view/international/englisch/travelservice/price_tourist.shtml