I'm guessing they had a bad time because they got pickpocketed :(
My tips for Venice - stay on Venice itself...don't stay in Mestre...for me (and it took until my third trip to realize this) - the best part was getting up and going out early - at 7am...to St Mark's Sq which was basically deserted - maybe a dozen people there (compared to what it looks like just a few hours later). No fighting hordes of people for a view of the bridge of sighs. Evening is gorgeous as well. We stayed at Al Campaniel B&B for 3 of our 4 stays. Very close to San Toma vaporetto stop and not too far a walk to Rialto - maybe 10-15 min, or a few vap stops.
And if the crowds at Rialto or St Marks are getting you down, just wander away from the crowds - about 5 min is all you need to get away from the traffic jam of tourists that happens in those areas. I loved just wandering the alleys and further reaches (as Rick says - you're on an island, you can't get lost!).
If you want to do the Doge's Palace, you can get a combo ticket at the Correr museum nearby (interesting in it's own right) that allows you to bypass the line at the Doge's. Our first visit, we stood in line at Doge's for at least an hour - precious time lost waiting. Burano is gorgeous if you are interested in photography (as I am).
Don't eat at the restaurants near St Mark's and Rialto - the price is high and the quality relatively low. My hubby and I aren't fussy, but one place along the main drag just past the Bridge of Sighs - we paid probably double for a pizza than we would have elsewhere...and we didn't get a coke (and I may have been hunger hallucinating) but I'm sure a bottle of coke was 10-12 euro! Drink water (or wine if you drink wine - we don't).
I found that even if the line at San Marco looks daunting, the most we've waited (in Sept) was 15-20 min - it moves fairly quickly. We did both St Marks campanile (bell tower) - first visit - which had a very long line - and the bell tower across at San Giorgio - 4th visit - absolutely no line, same views over the city, and amazing views over to St Marks Sq - all along the waterfront, including great views of Doge's and the bridge of sighs - which you don't get if you are at SM campanile.
But back to avoiding crowds - do go out early and late to the busy spots - they will be much less so. Use the afternoons to get off the beaten path away from the day trippers - head to Murano/Burano/Torcello if you want.
A vaporetto pass is your best bet for getting around - they seem pricey, but at 7 euro a pop (well, last year it was) for a ride, it very quickly pays for itself.
My funny 'holy crap this place is crowded' story - our first visit we were speaking with a merchant on Rialto Bridge (this was the 3rd week or so of Sept)...our comment was how many people there were (it was like sheep being herded)...his comment was - this is nothing, you should see it in July and August! UGH!