Grenoble offers some great roads reserved for pedestrian and non-motorized vehicles. Have a look at the Isère and the Drac banks. They present many, many kilometres of cycle paths. Besides, you will be able to observe beautiful landscapes during your ride. You can find some information on this website.
If you want a bigger challenge, some of the passes and climbs of the Tour de France (for instance the iconic 21 switchback climb up to l’Alpe d’Huez) are really close as well. Some of them are even closed to all motor vehicles one day in the summer. More information on that can be found on the website of Oisans col series.
You can also check out this website. It offers a few routes to follow. Although the pages exist only in French you should be able to download GPX or KML file to use with a GPS.
Grenoble is well known for having a lot of surrounding mountains. Some of them are stuffed with cable cars and function even during summer, allowing people to go up with a bike… Or a unicycle! These “summer” resorts generally draw bike paths and maintain them for riders. Feel free to go there and do some downhill or uphill.
For example, you could go to Villard-de-Lans, Les 7 Laux or Chamrousse. There are many more to discover; just look for it on your favourite search engine!
Grenoble is the big city so urban unicycling spots are legion. You won’t have any problem to find a place to ride! Nevertheless, you can check the campus which offers a lot of space. Besides, there is some skateparks. A new one is settled next to “La caserne de Bonne“. Another one is under a roof in “La Bifurk“. This one is very practical when it’s raining ! 😉