I think extra batteries are def a must with the gopro. I have had mine since 2013 (1st trip to disney using it) and have purchased loads of different accessories (no pole yet though). The accessories I couldn't do without are the lcd touch screen, water proof case, extra batteries, a charger that charges 2 batteries at a time, a clip mount (which I can clip to my shirt - it rotates too - great for indoor rides & walking around the park), a wrist mount strap, a skeleton frame mount (which is black & fits around the gopro & has a mount) & a lens cap.
Most of the mounts, clips, etc I purchased from Ebay with many less than $5 each (inc shipping), the lcd screen was the most expensive accessory I bought - but totally worth it. I could do without the double battery charger or the lens cap.