Steal Tim Nolan's design for a MPPT solar power management shield. There are a few solar things out there ("LiPo rider") but everything I've seen requires a single cell LiPo and support a similarly small voltage range for solar input.
A load cell shield. Search the forum for the number of people looking for solutions.
A shield for interfacing with piezo sensors would be nice too. Granted it's not hard to make a basic "knock" sensor, but if you want to massage that signal to detect light/hard hits
then a good, prebuilt shield would be great. Might lead to a lot of annoying Youtube videos of people building drum sets though.