Usually a new site takes up most of your bandwidth. But if you have the manpower, here are some things I would consider:
Cookie'ing visitors -- at least put the Marketo munchkin on your site so that you're not waiting 45 days to start tagging your visitors. The web histories should carry over, you'll just have to remember your old URL structure
Emails -- if you have an auto-responder system in place, it's probably not wrth re-inventing the wheel. But you should start now with any regularly scheduled emails you send. In fact, send more than usual if possible -- this would speed up the cookie'ing process mentioned above.
Start the learning process now(?) -- if your people are overwhelmed with getting the new site up (that's our situation right now) you may create a negative experience by asking them to learn Marketo now. But if possible, it would be great if your team had a basic understanding in place before your go-live. Our team was switching from another system they liked very much, so it was helpful to transition them to Marketo a little slower.
Forms -- if you will be using the same forms on the new site, might as well create them now.