First of all, the hackey sack. For beginning knitters, how about fulling it, which will compress it, and keep the stuffing in? My thought was to cast on 2 inches worth of stitches, knit for 2 inches, add the same amount of stitches on either side, knit for another 2 inches, decrease the side pieces, and knit for 4 inches. That would give you a piece like
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Then sew up the sides, fill it with beads surrounded with pantyhose, or something, sew up the top, and throw it in the washer. If you make it kind of tight, the sides will form around the beads, giving you the rounded edge.
If you want to do it without felting, I'd make it an inch on each side.
If you are a more experienced knitter, (I've been looking at this from Serena's viewpoint, who wanted to teach her new knitters by making these), I'd cast on 2 inches worth provisionally (I like the fulling idea, can you tell?) and knit for two inches. Then, leaving those stitches on one double point, pick up one set of side stitches (same number as one the other needle), pick up the cast on with a third needle, and then with a 4th needle, pick up the last side stitches. Knit in the round, purling the last stitch on each needle (for a corner). Do this for 2 inches.
Then, fill the cube with your beads (surrounded by hose or roving or something to keep them from escaping) just larger than you want the finished size.
Start the top. Knit on one needle only, and as you get to the end of a row, knit the last stitch together with the next stitch on the side needle. When you get to the end, graft, three needle bind-off, whatever, to get the end sealed. Throw in the washer, and let it shrink. Slightly overfilling the center will cause the bulges normal on the sack.