Exercise while pregnant

Hi ladies! So here in Michigan it is cold and snowy and I just am starting to feel down or like I need to exercise and feel good about my body and boost my mood to have more energy.  My doctor said I can exercise but said to not do exercises where you "bear down" or have my heart rate go higher than 140. I have always been active with my job since I am on my feet most of the day but I don't exercise regularly. I really want to tone my arms and legs and hips. How do I do this without losing weight? I also eat well balanced with fruits, veggies, grains, protein etc. a little extra for the baby but not a ton. Thank you in advance! I am 21 weeks.