Bicycle access options for a week-long stay in Paris
Velib makes the most sense for you. The use case you outlined is exactly what bike shares were designed for: short point to point trips where you don't necessarily expect or need to take the exact same bike back with you. 

Check out the prices: 



So, you'd take a &lt; 30 minute one way ride in the morning for 1 Euro, ditch the bike for the day, then take a (probably different) bike home for 1 Euro. 2 Euros round trip. That's probably cheaper than taking transit, and I challenge you to find a bike rental for &lt; 15 Euros daily. Even if you can, as noted in the comments above, you then have the additional worry of theft/ vandalism, and hassle of having to return it to a bike rental place when you're done. 

If you want to do a longer bike ride or full day of bike exploration before or after your conference, you could do a one-day proper bike rental for that.