"Most" trains do not continue to Santa Lucia. For example, I live east of Venezia in Portogruaro and we get just as many trains from Mestre as SL does, if not more (trains go up to Udine, Trieste, etc, so not all go to SL). That being said you can catch a nearly hourly train to SL or bus from the Mestre station.

As for luggage, in most larger stations there is a luggage check and storage station you can use for just a few Euros. Just be advised they normally close at 2100 or 2200 i.e. 9 or 10 PM, so have your bags back before then.

If you want a quick stop there are many trains via Bologna, where you can stop and walk over and get some food.

I fly and drive all over Italia, so I do not discourage flying as much as some here, however RyanAir would not be my first choice due to luggage, as some have already noted. And yes Treviso is a pain in the .... I never have problems parking or getting a bus flying into and out of Marco Polo, but Treviso is not as convienient. There is a bus from Treviso. I don't remember the hours, it is online somewhere.

AirOne (www.flyairone.com) flies from Rome Fiumicino to Marco Polo non-stop but it will probably be more expensive than RyanAir.

Ciao,
FH