A very common summer trip for us is Kenosee Lake in Moose Mountain Provincial Park. A weekend of wicked watersliding, camping, min-golf, beach time, and boating.
The Kenosee SuperSlides is fun for everyone with slides for all ages, there is a whole kiddie area with little kid slides and pools, tube slides, twister slides, bonzai slides, the free fall slide if you like wedgies, the lazy canal which is my ideal place to lounge on a floaty or air mattress and just relax while floating around the canal, and of a course a huge hot tub. There are lots of picnic tables and areas to leave your stuff and just relax the day away while sliding, I highly recommend bringing your own picnic lunch and snacks.
The Park campsites are decent in size, clean and nice. There is a great golf course, a fun mini-golf course, beautiful beach and lake. There is decent fishing too. And for those who like to be naturally adventurous there are lots of hiking/biking trails, I'm more of the sit back by the camp fire and drink kind of person so I couldn't tell you what the trails are like first hand.