We were in Toblach ten years ago in mid-July at the start of the MahlerFest concerts when they always do a symphony at the first concert. I highly recommend this exact time for anyone considering at trip to Toblach. This time (for various reasons) we went in August. BIG MISTAKE. The town was overun by Italians on vacation (the whole country is on vacation in August) and you would hardly know you are in a German-speaking area. I was able to catch the GM Youth Orchestra doing the Bruckner 9. The hut seems untouched from a hundred years ago, which in a way is nice. The surrounding area with the animal park (and kids running around) is a bit distracting.
The Meirnigg composing hut was a bit of a disappointment. First, it was closed, it is only unlocked a few hours a day for a few days a week. It is a long (10-15 minutes) hike up a heavily wooded trail. If there was a trail down to the Worthersee from the hut, it is now overgrown and inaccessable. The lakefront house seems a long way from the hut and can only be viewed from the road above it. All the houses along the lake road are chain-linked fenced-off, the owners are not interested in any "Mahler visitors". I guess you have to rent a boat to view the house from the lake.
The Steinbach hut is very well kept. You get the key from the Fottinger Hotel (where GM stayed) front desk and it's an easy five minute walk down to the Attersee (through a trailer-park!). The hut is immaculate with lots of exhibits inside, his piano, a message book for visitors, and even a speaker played the M3! The immediate surrounding area is well maintained with nice landscaping. The hotel even has some Mahler souvenirs for sale.
I'd gladly answer any questions anyone may have.