I have enjoyed trips by boat from AC to both Lamanai and Altun Ha. Both trips are really great experiences but if I had to choose the more rewarding one, it would be Lamanai. Keep in mind that Lamanai and Altun Ha are all day excursions with AH being a little less long.
Lamanai cost $125 US per person and included lunch at the Lamanai Outpost Lodge. Altun Ha was about $60 US and we had to purchase our lunch at a very nice restaurant close to AH.
Either trip is informative with lots of wildlife and exciting boat trips on the rivers. The guide I liked the best was Daniel Nunez who operates Tanisha Tours. The website is: http://ambergriscaye.com/tanisha/index.html/
Your hotel can contact him for you at no additional cost (sometimes less cost).
Hope this helps.
[This message has been edited by Sandcrab (edited 12-02-2000).]