To sleep there are hundreds of hotels, good hostel is the Yellow House, they make some mean oatmeal and spaghetti (!!!) for brekkie. Check out the parque central, on the side opposite the cathedral, behind a bookstore is lovely (though pricey) cafe condessa. Have a very North American moment at the Bagel Barn (off parque central) If you want to do a trek use O.X. Sky bar is awesome on 1 avenida, and there are some cool shops around there. I still love Fernandos (7 avenida) for coffee and early morning wifi.
I will admit that Antigua is Guatemala at its most organized and cleaned up. It is a pleasure to wander the streets, but is lacking the hard edge of the highlands or the funky soul of Peten. For me it was always all about indulgences: breakfast of fruit at the ceviche place, sammies from epicure, hair cuts on la septima avenida, pupusas at la rana y sapo, hanging with friends, and of course it is beautiful. If you can find your way to Santiago Zamorra to visit the weaving cooperative or out to Vahalla to the Macadamia nut farm, do it.
From Antigua most people head to the Lake usually via...
Most people end up here first and a lot of people don't like pana, but it is one of my favorite places as it is just a crazy weird part of guate. There is a fantastic little deli/restaurant on the main road with a tree growing in the middle of it, somehow i can never remember the name of it (algo de fuego or somethin'), but they have banana curry soup and it is SO good. Also there is fantastic Asian food at Chinitas (also on Santander) she makes some amazing blue corn pancakes and there is crazy live music every night. Oh and on your way out of town grab brekkie to go at Panapan bakery. The market right in town is fab, and they sell the wool clothes from Solola. Also if you don't want to go all the way to Chichi for a market you can check out the market just up the hill in Solola (grab a chicken bus). I'm not positive, but I think market days are sunday???
From Pana take the launchas out to other parts of the lake, they are going to rip you off on the launcha ride, but there isn't a lot you can do about it.
When you finally peel your self from the gorgeous waters of the lake head up the mountians to...
I like el cuartito for cute drinks and wifi, exploring mecardo Minerva for those plastic market baskets, and discovering all the hidden bars and restaurants. Oh and La Luna for hot chocolate, Blue Angel for vegan cookies, and the famous chocolate place near Mercado Flores (Doña Pancha), la fonda del che for peruvian flute music. There are always things going on at the gorgeous cathedral and lots of cool NGOs to support.
Anyhow everyone says it is amazing and very worth the long travel time to get there. So just go.
Amigos is a cool hostel, but tons of the hotels are very affordable, just make sure you get a fan and a place with a pool is always nice. Grab a launcha to take you to the mirador at sunset, bring beers and hike up to the lookout. Very cool. Lots of nice places to eat: Villa del Chef (killer mojitos which are 2 por 1 during happy hour 4-7), Cool beans for pancakes with fruit, and Las Puertas (near amigos) does nice veggie food plus the owner's husband plays classical guitar every night.
I think Tikal is done best if you head out there about 11, do a picknick at the grand plaza and then cruise around. By about 2 all the tour groups have left and you have the place practically to yourself. Pretty awesome. Sometimes you can catch sight of a toucan in the afternoon.
Obviously this is not an exhaustive guide, nor is this all that detailed, but it gives you a bit of a sense of Guate.
As far as getting around, the tourist shuttles are more comfortable but the chicken buses are far more entertaining and often a bit faster, if you can believe it. Spanish skills help out a lot but you will learn as you go.
Enjoy. Drink as much Zacapa on the rocks as you can, eat some pepian and make sure to give the Chapines lots of love.