This is a trip I’ve done a couple of times, starting from close to my apartment in down-town Grenoble (220m). It is possible to walk up from the city to the Bastille (460m), then pass round the back of the complex and access the monument to the mountain troops on Mont Jalla (614m), and continue onwards back over the top of Mont Rachais (900m if you don’t do the summit, or 1000m if you do).
You end up at the Col de Venice car-park (780m), where optimally you have a car waiting to take you back down to the city. If not, then it is a long and fairly dull walk back down country lanes. I’ve shown this main route in pink on the map below.
Coincidentally, Col de Venice is also the start of a trek up to Fort St Eynard (1300m) adding an additional 500m to the ascent (for a total of around 1200m). This path starts off steep, and remains steep until you reach the edge of the fort. Walking onwards for a few hundred meters takes you to the entrance of the Fort on the east side, and a restaurant plus toilet facilities. This optional extension is shown in green.
Update 27 Feb 2010: There’s a higher + longer route that takes you up to the summit. I did this today (across some large areas of frozen snow). I’ve marked this on the map in red. I also found a much more direct return-route that avoids going through to Col de Venice, which I’ve marked in blue. Including these two adaptations, the route came out at 17km, and 900m altitude gain/loss.