St Petersburg in 1 day?
 

Looks like I will have a day to spare ... is it worthwhile to try and get to St Petersburg from Moscow and back? There's a Sapsan train that arrives from Moscow at 9:20a and the last Sapsan back to Moscow is at 7:10p.

Also, will this create any problems with the visa? I am entering/exiting Moscow in sync with my visa application.

Thanks.

setting aside your visa question (not sure it will present any problems), had it been the reverse and you wanted to hit Moscow I would have done it. But theres so much to St Pete its not possible to do it even in a full day. I see it like going to a famous restaurant but not having the time to actually eat anything but to only look at what they serve

I think you should totally do it if you are okay with paying about $200 for train tickets to go for half a day.

You'll have enough time to do the city center of St. Petersburg: the Hermitage, the St. Isaac's Cathedral, the Kazan Cathedral, the Church of Savior on Spilled Blood. All these sights are within walking distance from one to the other.

if you really want it, why not? Not everybody has three days , whole week or even half day eating in a famous restaurant.

It would cause no problems with visas. I've only every stated one destination on a visa application (with a dummy booking) and visited quite a lot of places.

Have you considered a night train one way and the Sapsan to return? It would cost a similar amount of money and you could enjoy the splendor of the Red Arrow. It'd also give you a few more hours in SPB.
All of those sights recommended by daniitimin are on or very close to Nevsky Prospekt so would be ideal. But by taking the night train you'd have enough time to visit Peterhof which is a must see IMO.