First of all, I'm happy you feel great. That's wonderful. I really wouldn't worry too much about the weight gain. Once you are finished with chemo/steroids, you will begin to lose it. I must say I'm surprised your Onc would prescribe diet pills! Never heard of that happening.
I've lost nearly 15 pounds in the last 6 months by having a smoothie for breakfast or lunch every day! I make it in a blender (actually a nutri bullet) from a good handful of spinach or spring mix, a few strawberries, apple, pear, banana, blueberries, nuts, greek yogurt - basically whatever you have. You need the nuts and yogurt for the protein. Then add some water to liquify it and blend it up. It really tastes wonderful! Just experiment. You definitely need the greens and not just fruit (which is basically sugar). It doesn't look really good, but it doesn't taste 'green' either.
I also get to the gym 3 times a week. I do a little cardio and some weights.