hi yo Philly,
don't panic if the excursions to Russell have sold out. There will be a free bus from the jetty at Waitangi where the tenders should berth into Paihia and you will be dropped off at the wharf. It's a short ferry ride across to Russell from there....about 20 minutes, You can easily walk the town and visit all the sites while eating an ice cream and for a bit of excercise walk up to the top of the hill to the flagstaff and view the stunning bay from there. any questions just ask - I was bought up in the Bay and even though I live in Auckland I know it pretty well.
ps...there is plenty to do there, from the Hole in the Rock boat trip to visiting Waitangi treaty grounds and Marae which is a short walk from the jetty., there's a beautiful Kauri Forest and some good restaurants. Paihia is a beautiful beach town.