There is a direct shuttle bus ( Havaist ) from Istanbul Airport to Sabiha Gökçen Airport. You must ride the Pendik shuttle and disembark at the first stop of that line which would be Sabiha Gökçen Airport. This bus goes from the 3rd bridge so you would be in SAW within 50 mins after leaving IST. If you consider this as an option, I would suggest you to take the shuttle to Sabiha Gökçen Airport, check your bags and then go to one of the nearby hotels that offer a free shuttle from SAW. If you want to spend the time at the city, avoid Taksim, avoid Sultanahmet, avoid the Bosphorus line, avoid Sirkeci and avoid Üsküdar. In all these spots, weekends would be crowded with local visitors with cars in addition to tourists. However, the Ramada Üsküdar is not in Üsküdar, it's in Altunizade where traffic could happen in any unexpected time. Also the direct municipality bus from Altunizade to SAW is not operating anymore so with bags you would probably need to take a taxi from there.