Despite the official statements that this part of Ontario is a tourist destination at any season the best time to come is the summer.
Many attractions such as the forts are closed during the rest of the year or they will only open for larger groups after advanced notice. We made some disgusting experiences when we tried to show certain things to friends who came from Europe.

You will see everywhere wineries or signs which give you the direction to one. They all invite you for wine tasting which is not always for free. Also tours which familiarize you with wine making are very often advertised. I am not a big wine drinker or wine connoisseur. Either I like it or not. For that reason I do not feel qualified to make any statements about the quality of the different wines. You must do your own testing - but if you do too much of that you should not drive any more.
Also watch for the fruit stands. Local grown fruits of the season are offered here.
When you arrive in Niagara-on-the-Lake you will be immediately aware that you arrived in another tourist center. However, everything is much more tasteful than in Niagara Falls. No flashing lights are used for advertising and many things are higher standard. Obviously the concept to approach a different clientele is very successful and I can not understand why Niagara Falls did not adapt this concept at least for a part of the city, such as the downtown area. Niagara-on-the-Lake had already achieved a good reputation when I came in 1984 to Canada and in the meantime many improvements have been achieved. I believe a good investor is the key to this success.