I would suggest you build platform steps. Basically, two wooden boxes fastened together to form a pair of risers and landings. Use 5/8-3/4 inch thick exterior grade plywood for the landings and dimensional lumber of the appropriate size for the frame and secure with glue and stainless steel or galvanized screws. Its quite simple to construct and very strong. Here's a picture of what I am talking about.
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^^Can't argue with that. I can't think of anything better than including the reminder to use water-happy wood (like cedar) or make sure to seal the wood with an appropriate oil/varnish - and put skid proofing on the treads.
And maybe skid proofing on the bottom if it's not attached to the floor. Burf didn't leave much to be added.