What essentials do I need for spring camping in Ontario?

Ground pad/thermal break. Its going to be cold, hella cold, at night. I would bring a winter weight bag too.

Possibly obvious detail but in case someone reads this who doesn't know - remember to change your clothes when you sleep at night. Switching that sweaty tee-shirt for a clean one will keep you warmer and you'll feel less grungy in the morning. You might even want to do this earlier if your evening activities involve mostly sitting around after a day of being active. Main point is don't sit still in damp clothing.

Oh, and slight derail but related cold-weather camping tip is to remember to stay hydrated. You don't think about it as much as when it's hot and you're roasting but it is just as easy to get dried out and end up with a headache when it's chilly. Easier, in fact, since the humidity tends to be lower.

One more thing: just in case you don't already know, Algonquin park is both deer tick country and beaver fever/Giardia lamblia country. Deer ticks mean no exposed skin or loose cuffs: pants tucked in boots or gaiters. Giardia can be resistant to chemical water sterilizers. RO filters or boiling are the best options to neutralize the cysts.

moisture management is key! i much much much prefer wool to polar fleece when it come to winter camping. wool will keep you warm even when you're wet. wool socks, wool long johns, wool toque, wool t-shirt, wool sweater. and when i was camping last week, i really missed my thinner toque to wear at night. i was still cold (even with a winter weight bag and silk liner), until i put my (heavy, itchy) toque on.

As someone who has camped in 40 below, the most important thing about staying warm at night is a thermal barrier between you and the ground. A decent sleeping bag on top of that, and you should be fine.

Are you canoeing or just camping? If the former, make sure you have good waterproof packs (along the lines of http://www.mec.ca/AST/ShopMEC/Packs/WaterproofPacks.jsp). The only thing worse than wet clothes is a wet sleeping bag. Also, a good emergency firestarting solution is advisable, in case you need to build a fire in the rain (of which there could be a good deal in May). What part(s) of the park are you planning to be in?

nthing the thermal pad. As someone who has slept on bare earth in the middle of July (admittedly on an island in Boston Harbor) and woken up freezing, put something between you and the ground. Added bonus, it makes the sleeping surface softer. Extra socks. It may be just me, but I hate the feeling of wet socks, even socks that I have sweated in. Dry socks always make my feet just feel warmer. Oh and have at least some clothing/sleeping bags in a waterproof container. This goes double if you go canoeing (what I did in Algonquin), but just in case it rains on you when your camp is packed up.

You don't want cotton or any other moisture-absorbing material next to your skin, so leave the t-shirts and cotton socks at home. When I do spring trips I wear polypropylene long underwear and socks covering me completely from my neck to my wrists and toes. It has to be a material that wicks moisture from your skin to your outer clothing layers. Then a layer of wool over that, including wool socks. Wool retains its insulating properties even when damp (or wet). Over that you want a wind- and water-blocking outer layer with enough venting to let some air circulate and keep you dry. Also get a hat with a face net to keep the black flies away.