By plane
Rennes airport (http://www.rennes.aeroport.fr/) has budget flights to and from Southampton, Belfast, Birmingham and Exeter thanks to Flybe (http://www.flybe.com/), or Dublin and Cork with Aerlingus (http://www.aerlingus.com/which) has good offers. The airport is less than 5km away from the city center, and bus No 57 links it with the city.
Dinard/Pleurtuit/Saint-Malo Airport. One hour away, Dinard's airport offers other cheap options, with for example a connection to London with Ryanair (http://www.ryanair.com).
By bus
Rennes has also an international and local bus station, right next to the rail station. This is where you can get information about Illenoo see below and where buses such as Eurolines/
By Shared Ride
The cheapest way will be covoiturage or car-sharing. A lot of websites offer information about people wishing to share their car and budget. Covoiturage.fr, 123envoiture.com, or Allostop will help you out. Since 1968, traveling by car on motorways within Brittany is free.
By train
Gare de RennesThe easiest way to get to Rennes from Paris is through Gare Montparnasse. There are TGVs almost every 30 minutes and the ride is 2hrs and 3 min. Tickets are available on the SNCF website, and between 25 and 65 Euros for one way. If you're under 26 years old, and planning to travel in France by train, get the "carte 12-25" 49 ⬠which will offers you 50% off most of the time.
There are also direct trains, 4 a day, to Paris Charles De Gaulle airport, which arrives at Terminal 2, taking approximately 3 hours. The train to CDG terminates at Lille, taking 4 hours, from where it's possible to take a Eurostar to the UK or numberous connections to continental Europe.
The Rennes Train Station also provides train service to Nantes, Brest, Quimper, St. Brieuc, and other cites in Brittany.