Can I extend my 30-day tourist visa in Thailand after returning from Laos?

You can enter Thailand Visa EXEMPT ENTRY by Land BORDER crossing a maximum of 2 times EACH CALENDAR YEAR.

Each one can be extended for 30 ADDED days at your local Thai Immigration office .

You don't have any visa(s) you have free stamp entry <- those aren't visa on arrival, those aren't tourist visas, they are just free 30 day entry stamp (visa exempt entry)

You can get ONE 30 day extension PER 30 day entry stamp

SO if you're here now on a new 30 day entry stamp you can go to the immigration office and get one 30 day extension

You are allowed TWO free 30 day entry stamps by land in a calendar year (Jan 1 - Dec 31)

Yes, you can apply for a 30-day tourism extension on your current stamp which is visa-exempt entry.

As you entered without a pre-purchased visa for 30 days for free and it was eligible for a 30-day extensino for tourism purpose, you entered on visa-exemtp scheme. Not 30-day tourist visa (there is no such thing) or visa-on-arrival (which you have to pay 2000 baht fee for 15 days stay, and only avaiable certain 17 nationalities)