How to hem pants without sewing
This answer has been edited to include more detail.
PERMANENT HEM - The simplest, cheapest, quickest solution is to use glue. Make sure you are selecting a craft/fabric type of glue which is also dry-cleanable. If the glue is not dry-cleanable, just make sure the pants are never sent to the dry-cleaners or the glue will dissolve and make a big mess on the pants. When you apply your fabric glue, use a very thin bead, and follow directions for application, testing first to make sure there isn't a bleed-through to the right side.
There are also a number of double-sided adhesive transfer tapes available at sewing shops and some department stores. Some of these are heat-activated; be careful if you are using on heat-sensitive fabrics, such as synthetics and blends.
And if you are using any of the permanent adhesive choices, be aware that you may never be able to correct or change the result of your work without cutting off the part of the garment where this adhesive has been applied.
TEMPORARY HEM - I have had garments brought into my sewing shop to repair hems that had been "temporarily fixed" with permanent adhesive, either glue or double-sided transfer tape. First I had to use a throw-away pressing cloth and my iron to slowly and carefully loosen the glue so I could get the layers apart without damaging the fabric. Then I had to apply a separate thin fabric over the glued areas so I could re-position the hem and stitch it by hand, keeping rubbing alcohol handy to remove any glue build-up on the needle as I was working. It was not possible to use any of my sewing machines because of the presence of glue.
FOR A TEMPORARY FIX: Don't use glue. If you don't have a needle and thread, the best solution is to use a safety pin (make sure the point is smooth before sticking it into your garment, as a burr can cause a lot of damage). Other "fixes" should probably be avoided. Stapling can damage fabric. Regular tape and masking tape can leave a residue, depending on how long it's left in place before you remove it, and under what conditions (cold is better than warm), and the quality of the tape itself (some will leave a residue no matter how careful you are).
My advice is to always carry one of those tiny packets of clothing repair items which usually have an assortment of thread colours, at least one sewing needle, and a few safety pins.