The absolute best time for a ride if at all possible is sundays and probably nice and early in the morning. There are cheap bike (velib) stations everywhere and they are easy to use. Sign up on line and just use a code to pick up a bike. Check basics like seat and tyres before choosing one. I got a beautiful gold bike for my sunday ride but most are silver.
The banks of the Seine are the natural drawcard for an easy continuous ride. There are marked lanes along the roads or you can head down to water level and check it out from a new perspective. Beware of cobblestone sections they create heavy vibrations that take away the joy of riding and remind your buttocks that you are alive.
We criss-crossed several bridges and generally meandered in the direction of the Eiffel Tower. There are many quiet streets and pathways along parks that make it safe for less-experienced riders. If the traffic starts to get too busy get off the main streets and use back streets and lanes, or return the bike at one of the stations. Stretch your legs, go for a wander, grab a coffee and a snack and then jump on another bike ready for a new adventure.
A go pro camera is a great way to record some of your riding experiences in Paris. If you’re interested here is a short video on youtube: https://www.youtube.com/watch?v=K640TeouBHk