I go to pisa for about 3 months a year...so here are my top tips.
Ryanair always has a sale at some point or other...its just a case of having the nerve to hold on untill one comes up, obviously this might not be the best idea if you have a set date in mind.
Ryanair flights to pisa from the uk usually start on the 31st of march, after a period of not flying over winter. There is nothing in pisa worth seeing, apart from the tower and even that is overrated... go to florence instead. Train station is attached to the airport and a ticket to pisa central station is about 2 euros (remember to validate your train ticket at the yellow machine on the platform or you could face a spot fine from the train gestapo) From the central station you can catch a train to most places in italy, always remember to validate your ticket!...don't get a taxi whatever you do.
For really cheap accomodation around pisa there is a camping site not far out of the centre... Don't leave any form of bike outside in pisa over night. They descend like hyenas over night to rob pretty much any bike, no matter how locked up (yet some bikes appear to have been abandoned and chained up to bike racks for at least 5 years).
When i was living there i had 3 bikes taken...(all 3 of them weren't worth stealing).If your bike gets stolen you can buy yourself a cheap bike if you hang around outside the student canteen, it won't be long before a smackhead/student/illegal immigrant turns up trying to sell bikes for 20-30 euros (Expect the carrefour supermarket town bikes resprayed matt black or florescent pink depending what spraypaint was on offer that day.)
Bars are really over priced for beer...almost 5 euros for a pint although there are a couple of cheap places where a beer is a euro its just a case of finding them,the better plan is to buy booze at the supermarket where a beer can cost as little as 60 cents or stick to the red wine at 2 euros a bottle. Eating out on the other hand is cheap...so its swings and roundabouts.
As for driving to italy, yeah you can do it...but you better be good friends or else you are all going to want to kill each other by the time you get there. To be fair though if you share the driving you can do it in under 24 hours from dover.