My suggestions here assume (i) you are a first time visitor to Vancouver, (ii) you only have a day or two for sightseeing, or perhaps just a few hours between conference sessions, meetings, and socials, and (iii) you are staying in downtown, probably close to the Convention Center. If you have longer to spend here, there are plenty of rich Vancouver tourism resources that will help you explore the city in depth.

1. From the Convention Center you can easily access Stanley Park via the seawall. This is a must-see attraction. For the physically active, consider jogging around the seawall or renting a bicycle for a quick loop of the seawall (possible in < 2 hours). If you start out cycling, just be aware that the seawall is one-way for bicycles (counter-clockwise only). The seawall is almost entirely level, so it is accessible for wheelchair users. The views from the seawall are excellent; other highlights in the park are the Totem Pole collection and the Rose Garden.
2. If you have at least a half day to spare, visit Grouse Mountain by taking a free bus from outside the Convention Center. I advise first time visitors to pay for the gondola to take you up (and down) the mountain, though if you want some vigorous exercise you can walk up the mountain on the Grouse Grind (c.60 to 120 mins of staircase-like uphill hiking. Hiking down the grouse grind is not permitted, but there are inexpensive "download" tickets to use the gondola to get back down).
3. For a memorable lunch break from conference activities, take the SeaBus from Waterfront station (which is adjacent to the Convention Center) across to Lonsdale Quay in North Vancouver. It's a 12 minute crossing with departures every 15 minutes, and costs just a few dollars. Browse the Lonsdale Quay market and eat at one of the food stands.
4. If local foods and crafts are your thing, don't miss Granville Island Public Market, a classic destination in Vancouver that featured on Seth Rogan and David Chang's culinary tour of the city. You have two main options to reach Granville Island from downtown: either take the #50 bus down Granville Street and across the Granville Bridge, or take an AquaBus from the bottom of Hornby Street.
5. Did you know Vancouver has multiple beaches in the heart of the city? The weather in August will be ideal for dropping by the beach for a picnic, an ice cream, or a game of volleyball. The best beach for splashing in the water or renting a kayak is probably Kitsilano beach, which also has a large open air swimming pool you can enjoy. For picturesque views of the city head out to Jericho beach. If you can only spare an hour or two, try to visit English Bay at sunset. English Bay can be reached on the #5 bus which you can take from Howe & West Pender, just a few blocks from the Convention Center.
6. Another good excursion for a half day is Capilano Suspension Bridge. Like Grouse Mountain, the entrance to this visitor attraction can be reached by shuttle bus from outside the Convention Center at Canada Place. Enjoy the thrill of crossing the suspension bridge itself, and explore the Cliffwalk and the Treetops walkways to really immerse yourself in B.C. forest. If you like hiking, you could add on a walk in the forest just uphill from the Capilano Suspension Bridge attraction, called Capilano River Regional Park. And since they are both in the same area, 4km apart, you could consider visiting Grouse Mountain and Capilano Suspension Bridge as part of the same day-trip. And if you are a fan of forests and bridges, another good place to check out is Lynn Canyon Park & Suspension Bridge. This public park does not have its own shuttle bus, but is reachable by public transit by taking bus #228 from Lonsdale Quay (see item 3 in the list above).