Lago is my favorite restaurant on The Strip. I should note I don't eat beef so if steak is your thing, this is probably not the restaurant for you. Also, if you want a giant pile of food on your plate for relatively short money this is also not your place. But if you like to tapas/small plates style of eating or if you don't want to leave feeling like you might burst then go here.
Lago puts you eye level with the water show so ask for a window seat. It can be a bit loud in the restaurant during peak times (acoustics, not blasting music or anything like that) but if the weather is not too hot you can ask for outdoor seating and it is much quieter outside. It's the best outdoor seating on the Strip, and there's not a lot of outdoor seating on the Strip! Book in advance if you want window seats or outdoor seating. (Another good thing about Lago is it has a bathroom IN the restaurant so you don't have to walk out of the restaurant and into the casino. Which might not sound important, but it sort of is. Also, it is a really nice and private bathroom! Though, they do use sage scented hand soap which is the bane of my existence....)
I like to dress up and you won't feel overdressed at Lago if you want to get dressy. But you don't have to. Casino's will seat people wearing just about anything, so smart casual is fine.
I would get a reservation for any Strip restaurant. You can always cancel if you change your mind.
I love the food here. There's a sister restaurant in Aria- Jullian Serrano Tapas- which is equally good but is interior to the casino so has no views.