Best buffet is the Sterling Brunch at Bally's on Sunday mornings. $100 for all you can eat lobster tails, king crab legs, prime rib, lamb, fresh oysters, filet mignon, caviar, unlimited champagne pours, bloody mary bar, and all sorts of other goodness. You won't get a table this Sunday though. Or any Sunday this summer. They're booked until September. The bar is open seating but there's only about 25 spots and they're taken right at 9am.
Caesar's and Cosmopolitan have the best outside of that one but A.Y.C.E. is cheaper and getting really good reviews.
For a 3 day stag, pool parties by day, bars and clubs at night. Only good daytime show I know of is Mac King and, while great, I'm not sure he's really geared for a stag party. Absinthe would be a great show for a stag but it's around $100 a ticket. Of course there's the topless shows at Flamingo and Luxor.
Zipline at the Linq and then do the half hour open bar ride on the High Roller.
I wouldn't worry too much about the heat. So many spots have misters to keep it cool and you'll be in and out of places with the AC on blast while you're walking. Wear broken in comfortable shoes. Stop into the multitude of Walgreens/ CVS/ ABC Stores on the Strip and grab a bottle of water along with a can of beer and do your drinking that way instead of stopping at casino bars.
If you want to splurge on a cocktail, try either the Verbena at the Chandelier Bar in Cosmopolitan or the Sinatra Smash at Parasol Down at Wynn.