Kayaking has undergone big changes over the last 10 years. There are new designs coming out every year and older designs sell for cheap. If you're just looking for something for kids to play around in on the lake, you should be able to pick up old whitewater kayaks for $50-100. I bought two older designs for my daughter for $50 each. Cost me $50 to ship one from Salt Lake to Denver via Forward Air. New all round flatwater kayaks can be had for around $3-400. Most all recreational kayaks these days are made out of polyethelyne and can take a lot of abuse

Here's some examples:
http://denver.craigslist.org/boa/204795359.html

http://denver.craigslist.org/boa/205128291.html

http://denver.craigslist.org/boa/204041877.html

http://denver.craigslist.org/boa/190811670.html

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13