Aside from a great mobile, it's good to have other learning tools in the crib or hung from the sides of the crib that are viewable by baby. Some basic rules of thumb are to have items that have strong contrasting patterns and a variety of shapes. However, for safety reasons minimize how many items are actually in the crib next to baby. If you have items hung from the sides of the crib ensure that it is attached firmly and not left dangling so that baby can pull it out or get tangled. Try not to overdo it either because if things get too "busy" it will just be confusing for baby and may defeat the purpose.
A key item that should be at eye level for baby is a reflective mirror surface. Not the glass version though. Babies are very interested in reflections. You can purchase mirror card at most art stores that are great for observing reflections. A single first learning toy in the crib is sufficient for baby. Try not to overwhelm the space with stuffed toys. Aside from not being that stimulating, they are a safety risk and trap dust.
So that's the standard stuff, education-wise. What will make your sensory toy unique is the creative, personal extras. I made my own sensory ball and rattle and these were the only two crib toys I had for the first three to four months of my baby's life. A sensory toy does not have to be a ball. But the idea for a first sensory toy follows the same basic rules of pattern and shape variations, and can have mirror card along with parts that make different noises when knocked, grabbed or squeezed. So you can make a huge sensory block, a long caterpillar style sensory toy or a ball.
Angular shapes don't make much sense, as they won't roll easily when pushed. Although if you round off corners in a block it shouldn't be too bad. As your baby has little control of arm and leg movements initially, the rounder the better, so you challenge them to stretch and roll to make contact with their sensory toy. Stitch your sensory toy together with nice soft cloth that has interesting patterns. To cut back on the cost visit a fabric store and get end pieces that they are happy to get rid of so that you have different colors and patterns.
You can put in some foil paper in one section of your toy so that you get a crinkling sound when it is squeezed. You can also try putting in a piece of elastic in cloth attached to one side with a tiny "pompom" or stuffed animal at the other end so that it can stretch out when pulled. You can also attach plastic rings or put a bell(s) in the middle. Fill in the rest with soft cloth to make up the shape or buy some sponge for stuffing.
If you're into batik that has more abstract, less rigid patterns, then do your own cloth in your choice of colors for making up your toy. If you are artistic you can hand paint your own patterns. Crochet and embroidered surfaces are fantastic! Not throughout the entire toy but probably making up different parts so baby gets a sense of different textures.
You don't have to stick to being creative with just the materials, the attachments that you use on the toy can be unique too. Think recycled items (safety first of course!) and you can include old sentimental items too. This one is really about personal taste. A nice idea is to put in a "pocket" and insert a photo of mum and dad and cover it with some hard clear plastic. If you want you can put a cloth flap over this so that baby and you can do "peek a boo" lifting the flap to expose the picture and then covering it again.
The rattle I did was a lot simpler. A clear bottle filled with beads and sealed off with a PVC pipe. The key with this one is for baby to see the movement that is associated with the noises. Here you can get as creative as you like with the container, the items and colors inside the rattle. Just be sure the size and weight is realistic for your child.