I'm sure someone from the region will pipe in, but here's the thing -- Joinville Le Pont is a SE suburb of Paris, and as such, the only way you would be able to get to DLP on public transportation would be to take the train into Paris and then the RER out to DLP Chessy...Since park opening hours correspond with rush your, your talking 45 minutes into town by train, and then another 45 minutes out to DLP by RER -- 90 minutes is enough of a commute I wouldn't do it in two directions each day...so if you go for more than one day, then an overnight at DLP would be most beneficial (and you'll get Early Entry the second day)...BUT -- in reality, your best option is just to drive right down the A4 and then North to DLP in Chessy -- its probably a 30 minute drive if you have a car instead of a 90 minute commute
The best days to visit are Mondays and Tuesdays -- weekends are packed all year and particularly packed during the summer. In June you won't have to worry about French school holidays -- though you will from other European countries and the UK -- Wednesdays should be avoided at all costs -- kids in France don't have school on Wednesdays and the parks are packed that day. Absolutely the best days to go are days that rain on Mondays and Tuesdays -- locals will avoid the parks like the plague and they can be deserted.
If you have been to WDW, you follow a similar touring plan for DLP -- start with Space Mountain at park opening and cross back to Big Thunder Mountain and then pick up all the other attractions afterwards in whatever order you want. Pirates and Phantom Manner are lightyears better than their US equivalents so don't bypass them. Star TOurs is still running the old version, so you can skip it. There are some cute Fantasyland attractions we don't have in the US....but overall you should be done with DLP by 2:00 or so and then head over to Disney Studios -- the studios do close earlier than DLP in most days, though -- but their headliner attractions (Tower of Terror, RockNRoller Coaster, and Crush's Coaster are all doable -- what you would miss out on with a one day visit is the smaller attractions and the shows -- Cinemagique and Animagique are excellent with no US equivalents. Skip the tram tour - its as bad there as it is in the US. The car stunt show is better, and more creative in the original French (with instant english translations using Disney Speak)...
You don't have many dining options, not like the US parks. There are a couple sit-down restaurants but they are not great. Disney Village offers some other options as well and just opened an Earl of Sandwich too! All ticket information and hotel and dining information can be found at
http://us.disneylandparis.com/index.xhtml
SInce you don't say where you are from, I am going to imagine the US....if you are from the US, you can currently get online a 2-park parkhopper ticket good through the middle of October for only 59.00 each -- a good amount off the regular one-day two park hopper -- its a special promotion good only for US residents, so if you are from the UK, sorry, youi'll have to check to see if there are other promotions going on. A two-day park hopper will discount your price even further for just a nominal extra charge. The website has all the information......also check the forum in here purely on DLP posts...