Princess and Holland America have long held the largest share of passengers -- at least traditionally -- in Alaska's waterways and land-tours -- but ever- increasing competition continues. First, of course, were cruise lines, from Carnival to Norwegian to Radisson Seven Seas to Seabourn to Crystal -- that not only debuted service there and then added full-season sailings not to mention cruises that featured, often, its newest, swankiest ships. Now, Celebrity, which with sister cruise line Royal Caribbean has been further developing land tours along with its seabound trips, announces a healthy jump for next season. Royal Celebrity Tours, the combined land-vacation company, will offer 20 cruise-extensions next year; it had 13 in 2002. In addition, the company, which primarily offers two-night packages focusing on Denali National Park, will create an expanded three-night version that includes a full day excursion on the Kantishna Wilderness Trails -- 95 miles worth of prime Denali wildlife habitat.