Swimming's definitely a good one for toning without straining the knees too much! Only trouble is I'm 8 miles from the nearest pool and by the time I've spent petrol and entrance fee it's costing me about £8 a time... I get bored in the pool after about half an hour so it's not very cost effective! I think in the winter I'll join the gym there as you get free swimming and group classes with the membership and I'll get a reduced student rate, so it will make it worth going more (and save me braving the icy roads!).
I'm chuckling at your 'run envy' as I used to feel the same about everyone I saw exercising. I've had an orthopaedic problem with my feet all my life and it's caused problems with my stability and my knees/hips/spine are all out of line. Just walking the dogs made everything hurt for days after. So goodness knows what I was thinking when I suddenly had the bright idea to go running... insane. But it's forced me to get everything tested and get the right supports and stability shoes etc. and to do all the muscle exercises I've been taught (but never bothered doing) to strengthen me up and get everything back in line. Just ran a 5k yesterday and I have the least knee pain I've ever had in my life - which feels very strange! I still can't believe I couldn't walk up the stairs without stopping for breath just 6 weeks ago. I don't care if I never lose another 1lb in weight as long as I never, ever let myself get back in such a state of unfitness again!
But sorry, back to swimming and classes - I think that's a good idea, even if you just have an extra shake before the exercise so that you've got a few carbs to burn while you're jumping around, you'll still lose the weight. I worry about loose skin too, especially the belly. Mind you, after 3 children and a c-section it's never going to be bikini-ready however much toning it gets!
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