I'd love to see an expanding panel of favorite items available on any page. In particular, Power-ups would be great - I could easily use one from anywhere on the site without having to leave the section I am currently browsing. Bad-guys would also be nice, and the other sections could have merit as well.

I envision a simple "empty star" icon next to items like Power-ups, Bad Guys, etc. Clicking the icon changes it to a "solid star" icon with a "favorite added" message. (I'm borrowing from the way Chrome lets you mark pages to bookmark and how the icon works.) Clicking a favorited item's star removes the favorite status.

Once an item is starred, it would appear in a panel of favorite items, accessible on any page. A star icon could appear next to the Secret Lab and Power packs icons. Clicking reveals a panel with favorite items, sorted by section. I'd want to see a list of the names and buttons to quickly mark "I did this" or "I battled this" with any associated popup.