We've been to Paris many times. Since people have already answered your Velib questions, let me add to your wifi decision as we just got back from a 2-week trip in September-October and tried for the first time to do the wi-fi only thing.

It's do-able, but don't rely on Boingo. I got a Boingo subscription and it was worthless. The only places I got wifi reliably were our apartment (obviously) and in cafes. Eventually we gave up and added data roaming to one of our phones. Totally worth it for data on-the-go.

For voice, I put $5 on Viber and made calls over wi-fi. I made half a dozen local calls and 3 calls back to the US. One of those US calls was to a land line and lasted 13 minutes. Of my $5, I used $0.59 to make all the calls. The call quality was crystal clear; better than cellular. This, IMO, was a much better way to go than buying international minutes.