Can a portion of a back yard be leveled off fairly easily for installation of back yard items?

My yard is sloping mostly on about a 30-45 degree angle in places. Makes it really fun for water slides and sleigh riding but a pain for mowing and other necessities. I have thought about having a concrete slab poured for a small patio and would like to have a pool put in. There is a portion in the top part of the yard that is almost perfectly flat, but it is not enough to put everything. There would be enough room for MAYBE a swingset for the kids considering the other portion is filled with a flowering dogwood and some other beautiful flowers which will not be removed at this time. Poor landscaping choices from the previous owners of the house. How difficult would it be to correct the slope of the yard? I don't see any possibility of getting any kind of heavy equipment brought in here. The grade on the street above me would be entirely too steep. There is no room between the house beside myself and I to move anything like even if we removed the fencing temporarily to do so.