The Short Answer
The two most romantic things to do in Las Vegas without buying a show ticket are the gondola ride at the Venetian and the Eiffel Tower Viewing Deck at Paris Las Vegas. One sets you at water level behind a gondolier who actually sings; the other puts you 460 feet above the Strip in open air. Both are European landmarks rebuilt in the Mojave, and neither substitutes for the other. Here they are in order of altitude, water first.

Gondola Ride at the Venetian: Water Level, Serenade Included
The gondola ride at the Venetian is actually two different rides, and plenty of couples book one without knowing the other exists. The indoor route runs through the Grand Canal Shoppes beneath a permanently blue hand-painted ceiling, climate-controlled and smooth. The outdoor route glides along the lagoon at the front of the resort facing Las Vegas Boulevard, which is the better one for photographs and the harder one in August. Your gondolier serenades you either way, and Venetian tradition says you kiss under every bridge for luck.
Here's the detail that catches couples out: a standard gondola seats four, so a party of two on a busy afternoon can end up sharing the boat with strangers. If you want it to yourselves, buy all four seats or book the private gondola outright. And while you're in the Grand Canal Shoppes, the gondoliers march the indoor canal twice a day singing in unison, which costs nothing to watch and is the best free thing in the building.
Where: Grand Canal Shoppes and the front lagoon, the Venetian | Style: Seated, sung, water level | Ages: All ages, kids ride with an adult

Eiffel Tower Viewing Deck: 460 Feet Over the Strip
The Eiffel Tower Viewing Deck at Paris Las Vegas sits 46 stories up, reached by a glass elevator, on an open-air deck that runs a full circle around the tower. It was never meant to be half-size. The original plan was a full-scale copy of Gustave Eiffel's tower, and the reason you're standing at 460 feet instead of roughly twice that is the airport: the resort's proximity to what's now Harry Reid International capped the build at half the original's scale. It still took more than 5,000 tons of welded steel when it went up in 1999, and three of the tower's four legs come down through the casino floor, where you can walk right up and put a hand on them.
What the climb buys you is the one vantage point on the Strip that looks down on the Bellagio Fountains rather than across at them. The deck is open grid underfoot and gridded at the rail, so it is genuinely outdoors, wind and all. Best suited to couples who'd rather see the city than sit inside a themed version of it.
Where: Paris Las Vegas, just south of the Strip and Flamingo | Style: Open-air, 360-degree, 46 stories | Ages: All ages
When the Answer Changes
Summer flips this. From June through September the outdoor gondola and the open deck are both fully exposed, and a 110-degree afternoon turns romance into endurance. The indoor gondola is the only one of the three routes that's climate-controlled, which makes it the July default. Accessibility flips it too: the indoor gondola is wheelchair-friendly with staff assistance, the outdoor gondola is not, and the tower's elevator takes one wheelchair guest at a time. And if either of you is uneasy about heights, the deck's open grid is not where you want to discover that.
The Practical Truth
Water level with a singing gondolier is the Venetian; 460 feet with the fountains underneath you is Paris Las Vegas. The timing thing nobody mentions: the deck can close on short notice in high wind, so if it's the anniversary centerpiece, don't save it for the last night of the trip -- leave yourself a spare evening. Check the booking section below for current availability on both. And if you'd rather your romance came with a curtain and assigned seats, see our best Las Vegas shows for anniversaries.