We went last Spring with our 3 year old and had a fantastic time, (without EuroDisney). We found that the cheapest option for us was to rent an apartment for the week and found a lovely one in Le Marais through Parisattitude.com, (also meant that we didn't have to eat out all the time). We did a lot of walking and noticed that the city is peppered with little playgrounds. Parc du Luxembourg is great - little boats on the lake, a fab children's playground, pony rides, a puppet theatre and old fashioned carousel, all in a beautiful setting. Les Tuileries is equally child-friendly. I think they also turn the Seine into a bit of a beach during the month of August too.