If I travel during a regular work day (M-F) even if the time is outside of normal hours, I count it as a regular work day, and get paid as normal. If I have to travel on a weekend day (eg fly Sunday because I have to be someone at 8am on Monday) then I have the choice to claim additional time (similar to overtime) or take that day as a vacation day in the future. If it is my choice to travel on the weekend (e.g. I have to be there by 2pm Monday, and there are options for Monday morning travel) then if I choose to travel on the Sunday, there is nothing additional due to me.
In terms of spending the weekends away, to me it is absolutely a perk - to my good friend it is hell on earth! She has a little boy she misses terribly, and hates being in a strange city on her own - she would stay in the entire weekend, order room service, and skype her child! That makes it tricky to compensate appropriately - the best option I have seen is when I was working at a consulting company, negotiated for a long term contract with a client - you could either a) fly home and back again or b) apply the cost of the flight to staying in location (e.g. you could go on a day's sightseeing on the company's dollar) or c) you could fly to another location of your choice provided it was the same or less than flying home.
If this is just a one off, or infrequent occurance the for the junior employees, I would be tempted to keep it informal - maybe say to them 'we understand it is a weekend away from home. To be clear, we don't expect you to be working those two days, and if you would like to take a tour of the city, go to x attraction, we will cover the cost'.
Of course, if they are expected to work, or they are going to be catching up on work from their home office on the weekend, I think they should be compensated with overtime or time off in lieu.