definately pace yourself!! we had a blast in vegas last year. gambling was fun because we were together. we mainly did the slot machines lol.
we stayed at the mgm and watched the lions for a bit. we also saw ka by cirque du soleil, wasn't really impressed but i've heard that mystere is really good. i do regret not going to a burlesque type show, it would've been a fun couples thing!
we went to the hoover dam and grand canyon for a day...this was AMAZING but I wouldn't recommend going in one day because it's like a four hour drive and we didn't get to enjoy the canyon as much as we'd like. there is the sky bridge which is about an hour from vegas. it's supposed to be awesome to walk out onto the canyon on glass, but i remember it being really expensive for the experience and FI and I really wanted to see the biggest part of the canyon. might do the sky bridge when we go back.
definately stop and enjoy the fountains at the bellagio and the gardens as well, beautiful flowers. also, make sure to have dinner at night at the top of the eiffel tower, it overlooks the fountains and light show, we missed it, but heard it's extremely romantic.
also freemont street is really cool, make sure to stay for the light show at night it's worth it.
good luck