A friend had suggested http://www.venice35.com/ but I haven't booked with them so I cannot speak from experience. Also it depends on what your travel style is (hostel vs. apartment vs. 3/4/5 star hotels).

You absolutely must stay in Venice. This is an inexpensive budget hotel recommended on this forum. I'm considering it for our next trip to Venice.

http://www.alcampaniel.com/

Not a B&B.. but a really great place in Venice..

My sister and I stayed at this apt.
http://www.knowital.com/veneto/venice/cafoscari/apartment2.html

Maybe a bit above you price point but may fit your request.

Also I have stayed at http://anticocapon.altervista.org
My room was across the Campo in another building.

Submitted too soon..

Wanted to say in Campo Santa Margherita there is a bar called
DeChamps. Go there in the early evening for Spitz Aperol and potato chips !!

By all means tay in Venice - even if you have to compromise on the rest of your trip!

As other posters have said, you should defintely stay in Venice rather than take day trips there. I loved Venice at night even more than I loved it during the day! Once all the day trippers had left, the city was quieter and more atmospheric. It has to be experienced to be believed I think!

We stayed here: http://www.hotelvecellio.com/ and depending on what time of year you go, it may fall into your budget. The hotel is VERY small, but we found it very comfortable. Also, as it's based on the Fondamenta Nuove, we got a view of the lagoon and there was a handy vaporetto stop just outside. One of the things I loved, was that because it wasn't right next door to all the major sights, we got to walk through fascinating areas in order to get to San Marco etc. Only 5-10 mins walk (depending on how happily lost you get!).

I don't know how long you've allocated for the Lakes part of your trip, so assuming it's 2/3 days only, I'd suggest Lake Garda. Stay in Sirmione if you can, but anywhere in the Lakes will be expensive if you stay in a hotel. Guest house type accommodation will be cheaper, but not lakeside located.

Try tripadvisor for a list of likely places to stay.

I don't think the lakes are expensive if you do your research and track down B&Bs. All will be clean.

There are many other places to stay on Lago di Garda than Sirminone. Peschiera del Garda is also on the train line, with ferry connections to other towns on the lake. If you like pretty churches, go to Bardolino on Lago di Garda.

Bologna can be expensive (it's a business town) and be sure to book in advance. Be sure to enter the church of Santa Marie della Vita for the artwork, the complex of Santo Stefano is marvelous, and so is the Anatomical theatre and Biblioteca in the center of town. Ravenna, a train ride away, is a fantastic destination for art lovers.

If you are going to Lago Maggiore, I think Isola Madre is the most interesting island on the lake. Also, at the bottom of the funicular that goes up into the mountains (it's on the lake promenade) is a wonderful local lakeside hangout/bar called L'Idrovolante, where you can also get inexpensive homemade lunches.

On Lago di Como, if you are looking to hang out with the locals, try Menaggio for the passegiata.

Given your description of what you most enjoy doing, you might consider spending less time on the lakes and more time in Bologna/Emila-Romagna, visiting Ferrara and Parma.

What time of year?

Definately stay in Venice. We stayed at Locanda Gaffaro. It might be just above your price point, but I would recommend it. You can walk there from the train station. It is easy to walk anywhere in venice, but the vaporetto pass works well also.

I would absolutely stay in Venice proper. Have you looked around the train station area? I believe there are moderately priced hotel in that neighborhood. I loved www.lacalcina.com but it's well above your requested price range of 75 euro.

Besides your hotel room Venice doesn't have to cost much money. Just wandering around the city everyday is amazing.