I've paddled the Spanish in early May, Late May, July, August, September & October. I try to avoid June as a general rule due to the bugs. It's a very nice trip which includes a bit of everything. There is a ton of info available (YouTube, My CCR etc.).
Logistics are pretty simple, shuttles fairly cheap and there are a number of options in terms of trip length. Two primary options, the East branch and West branch, the East is the easiest, smaller rapids and no portages. The West usually involves a train ride to access Biscotasing Lake. The two branches join at The Forks, downstream of here there is plenty of reasonable current and a good selection of Class II rapids with a couple of Class III which with one or two exceptions can be run. These days when I paddle the river (East) I only do one short lining and one 75m portage, everything else can be run under normal water levels. The West branch usually involves a couple of portages upstream of The Forks, the number would vary on your ability to run the bigger Class III's. Speaking of water levels, there is a dam at the exit from Biscotasing Lake which controls the water levels downstream, because of the dam water levels are hard to predict but they are rarely way too high or way too low, there really isn't a time when levels cause any major issues.
Campsites for the most part of well used, clean and quite nice, scenery while not spectacular includes a bit of everything, it's especially nice in the Fall given a relatively high amount of deciduous trees. If you paddle in July or August you will encounter other paddlers but it isn't likely to be "crowded". early and late season it's possible you would see no other paddlers.
Where possible (time) I like to paddle the whole river, from Duke Lake (east branch) or Bisco (west branch) to Agnew Lake, a modest pace takes 6 or 7 days, it 145km for the East Branch, slightly longer for the West Branch.
The only downside for me is that the Spanish is now an official waterway park so fees apply.
