Not everyone lives where they can let their rabbit run freely about their home or yard. Here are some pictures of what I have done.
One design is a 4 ft square dwarf/baby playpen. I used 3/4 pvc and 1" grid wire for the frame and sides, the sides are 2 ft high. I used 4 wire panels from a 2'X4' XPEN for the bottom(2 panels) and lid(2 panels). I put 2 pvc braces across the top to support the lid. The bottom has to be wire so the rabbit can't dig out and a lid so the rabbit can't jump out. The sides are 1" wire so the dwarf/baby rabbits don't wiggle out. You can use more 2'X4' XPEN panels for the sides as well but some dwarf/baby rabbits can wiggle out through the gaps. The panels work fine for larger rabbits.
I used plenty of cable ties to hold the wire in place as well as the floor and top. The floor will lay nicely into the pen, cover with a fun medium, I use straw and timothy hay. Throw in a few toys, bowl of pellets or food if not using hay, and water. The lid panels are "hinged" with the cable ties so one or the other side can be opened. The lid can be fastened down so a crafty rabbit can't open it. I use springs from the hardware store to secure the lid.
email for more precise directions: LAVAROCKRABBITS@GMAIL.COM