I am here on visa exempt and will extend another 30 days. After that I would like to leave the country for a couple of weeks, apply for a 2 month tourist visa (can I get longer) and return.

I have a reasonable budget of about $100 a day for hotel and food. I would like to stay in a safe place where I can relax, hang out in coffee shops etc.

Where's a good place for me to go? Is it easier to go to Siem Reap, Laos or another place. Does one place offer a different length of tourist visa or are they all the same?

All tourist visas will be 60 days that can be extended for 30 days for most countries. Laos may not fill two weeks unless you travel at bit to Luang Prabang or such. If you just want to stay in hotels/bars Cambodia might be more attractive.

Go to Ho Chi Minh city in Vietnam.

If you're looking to get something for longer than 60 days, you might be able to get a double entry tourist visa in Laos -- I got one at Savannakhet earlier this year, and I think Vientiane issues them as well. (Plus I recently got one at Hanoi). You could get the double entry to cover you for close to 180 days, though you'd have to leave the country at some point in the middle of that period, plus do a couple extensions.