Santa Cruz la Laguna is really nice and is the place you go along the lake if you want to go diving. I went diving there and it is difficult. The visibility can be absolutely terrible depending on the weather and the rain. You have to go fairly early in the morning because the wind kicks up in the afternoon, stirring up the sediment and making it more difficult to go diving out of the little lancha. There's also not a lot to see in the lake, and coming off a dive at altitude and walking up the hill to the dive center can be challenging. At least it's cheap, and La Iguana Perdida hostel is sooo great. It gets cold there. Pay the extra few bucks to stay in the new dorm with 4 walls. Of course, things may have changed since I was there.

Tikal is awesome. Get there after 3 or whatever time it is that you get free admission the next day, and go late and climb up to the top of el Templo del Mundo Perdido and watch the sun set. If you do, make sure you have a headlamp or torch so you don't get lost. I've wandered around in the dark twice before and it's not fun. You can camp cheaply at Jaguar Inn, but if you want a room I suggest Tikal Inn because it's got a heated pool. A heated pool when you're backpacking is really cool. Lights and power go off at 9-10pm so make sure you have candles and prepare to sweat with no fan. Get up early the next day and walk around for another few hours. Make sure to see the Mirador tree and go looking for the crocodiles in the pond on the road that goes from the ruins to the hotels.

I've never heard anything special about the Pacific beaches, but a lot of people like the vibe on the Caribbean side at Livingston. From there you can hop on up to Belize by boat. Another spectacular thing to see is the waterfall at Semuc Champey. Antigua is also a lot of fun. The city itself is pretty, there are some volcano hikes nearby, and the food is rather continental while still traditional, and really good.