What do you mean you’re not sure if there’s plenty to do in Venice......Trish, highly recommend that you carve out some reading time in your guidebook, become more familiar with where you are going and make notes of what interests you. How do you know there’s a lot to see and do in Lucca? I’d allocate my time according to my plans in each place and the type of traveler I am. One can happily spend 5 nights in Venice if one does some homework, stays off the trodden tourists paths of San Marco and the Rialto and includes unstructured wandering time (with a good tourist map).
Note that Venice is extremely popular for mass tourism and as a logical result it is particularly budget-unfriendly, especially as for accommodations. If you don't have any enormous motivations to go there, just don't go, or don't go for five days (which are not so few).
You don't specify exactly what you want to do or to see, but there are lots of less touristy cities in Italy which would allow a positive travel experience, offering lower prices, less touristyness and more authenticity than the most popular ones. Examples in Veneto (which would also allow an easy day trip to Venice) are Verona, Padua and Vicenza; otherwise, for instance, Bologna, Ferrara or Parma in Emilia-Romagna.