It would help if you'd define what your style or interests are. Austin is REALLY eclectic -- whatever your interest, there's probably something for you. But, where I would take my parents from out of town is not typically where I would go with my husband or friends. KWIM? I am a huge foodie, so love all the different food options here. But, to be honest, some of the best food options are kind of off the beaten path or outside the city center, so I'll stick to those that are easier to find
If you want a really amazing (i.e. EXPENSIVE, but crazy worth it) anniversary dinner, Uchi or it's sister Uchiko for high end sushi fusion. If you want to take out a small loan (no...seriously), do dinner at Congress. It's really just an experience.
To have a cool/nicer experience, but spend less than at dinner, we like Lamberts for happy hour or for brunch. For an exceptionally funky experience, try High Ball. It's a 50's Vegas lounge looking bowling / restaurant / private karaoke place (adults only).
For bbq, personally we think Green Mesquite is some of the worst bbq ever. A better choice is going to Driftwood for the Salt Lick. It's a drive, but you'll drive through very pretty Hill Country. Just take cash and prepare to wait. It's not the best out there, but worth the experience. Honestly, we also don't care for Shady Grove, though it's an institution. It's a neat atmosphere if you sit outside, fun for kids, but the food is marginal (I've had better at Chili's). I do enjoy Threadgills, though I'm mostly saying that for the Blackberry Cobbler and bread basket
! For tex-mex, we always find that Trudy's continues to impress our visitors of all types -- Suiza chicken enchiladas and Mexican Martini that will knock you down
Like a PP said, you can't miss Whole Foods...either pick up stuff for a picnic, or just have dinner there. In addition to regular food stations, you can have a sit-down-with-a-server experience at their Fresh Seafood Counter, their wine bar or their Italian bar. But what we love to do is pick up various premade foods, a bottle of chilled wine, and take it up to the Whole Foods roof for al fresco dining and a skyline view (a lot of people don't even know it exists!). If you have a child, there's even a playground up there! Don't miss the bakery, b/c ooooooh myyyyyy ggooodness. The best around. Get a slice of the Chantilley Cream Cake...trust me on this
For breakfast, well, breakfast tacos at one of the jillion trailers, or good ol' sausage, gravy and biscuits at any of the Dan's. Though DH really loves the chicken and waffles at 24 Diner.
Watching the bats is fun...you can even buy a ticket for a bat boat tour. Also canoing on Town Lake or swimming at Barton Springs is fun. I would really suggest tubing in New Braunfels over Schlitterbahn, but that's best for a weekday than a weekend. New Braunfels also has the most amazing park, Landa Park. They have a natural spring pool (similar to Barton Springs), paddle boats and just an overall pretty park to visit.
I'd always suggest that if you're comfortable with driving, get lost in the Hill Country. The views are amazing, and it's fun to do winery tours (here's a website with a map of wineries:
http://www.texaswinetrail.com/ ). For hiking, try Pedernales Falls in Johnson City or Enchanted Rock outside Fredericksburg. In July, many peach stands should also be open, so look for ones that serve fresh peach ice cream. Yummo!
Was surprised to read a snake warning. Can't say they're any more prevalent than other areas. Now fire ants on the other hand...
It sounds terrible, but you may be really disappointed with Sea World if you've been to another location. Very small, not well-themed, sad to say. Even the Six Flags pales compared to others. Now the River Walk is always a nice visit, and can be very romantic at night for your anniversary.
Just a word of warning..DON'T ignore the signs for towing. It's sort of a young newbie right of passage to get towed. Don't even think you can get away with a minute or two. They circle like vultures. And note that a lot of roads downtown are one way...but not all. It's like a game to make sure you don't drive the wrong direction.
And for the love of pete, don't let anyone tell you 6th Street is good for getting a drink. Unless there's a band you really want to see or you are visiting The Driskell, Alamo Drafthouse or Parkside, it's mostly underage college kids and drunken craziness.
Hope you have fun! Let us know if you have any more questions. Austin is a great city
My parents Winter in that area, but by April or May they'll be headed back North to get away from the heat & the snakes 
Anyway, like I said it's a relative thing 
