It all depends on how often you'll need to carry things. A backpack might work well, but if it's warm out (and even if it's not), be ready for SBS (sweaty back syndrome).
I will suggest you check out a Topeak rack and a matching Topeak rack trunk. The
Topeak Explorer rack is a solid, basic rear rack, and it pairs nicely with a Topeak rack trunk with MTX mounting system. You could get a
rack trunk with fold down panniers. That way if you have a lot to carry you can zip out/fold down the panniers. If you're not carrying a lot, you could leave them zipped up. Or you could use just one pannier. The MTX stuff from Topeak is nice because of how easy it is to attach to the rack and take back off (all securely). The items linked above are what I've been using for commuting every day for about a year. I've been very happy.
Whatever you settle on, congrats on the new bike and happy riding!
