There is actually quite a bit of water in the ATM tour. You are swimming/wading on & off through most of it. Plus it runs aboiut $85 pp from SI so it would take a big chunk of your cash. $300 for the week will not get you many tours but there is a lot you can do on your own and if you travel by bus you can get around cheaply. Cahal Pech ruins are just on the outskirts of SI town and you can walk there. Che Chem Hah Cave is not too far and cheap entrance fee once you get there (not sure if you can bus there). You can rent canoes in SI town and canoe on the Macal or Mopan River. Take a bus toward the border & get off at the road to Clarissa Falls, walk down the road, have an excellent lunch and then you can hang by the river or rent tubes and float pretty inexpensively. The zip line is done from Jaguar Paw resort, near the zoo (you can get off the bus at the entrance road but it is a long walk in) and runs $55 pp. Tikal is probably out of your budget.
Where are you staying?

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