Great trip, we did the one day Crater Rim which apparently they only do once every couple of months as most people prefer to camp up[ there. We were happy with our decision, nice to get home to a shower (and beer) at the end of the day and you have to come down anyway ay ! The toilet sich up there didn't look that appealing...
Our guides (Anton and his brother Daddy) were excellent and knew the mountain like the back of their hands. Lunch was great and they provided plenty of water for the walk.
Take your passport, no one told us we needed it but you do in order to get into the park on the day, which is strange and they are also a bit pedantic about it, my passport number had changed since I booked the trip which caused an issue (resolved by the guides after half an hour on the phone.
Take them up on the offer of poles, very slippery on the way down and they make a big difference.
Make sure you have decent shoes with proper socks that keep stones out (not cutesy little anklets like my daughter wore)...and toilet Paper and an adventurous spirit if you think you may need to go...
For the record it was 1,850m of ascent over 9km's on the way up for us which took 4 hours and then 9km's back down (obviously) which took us 3.5 hours, very slippery so we actually found this harder. Your in the shade for 3/4's of the way up and back, so its only sunny for the final push, and at that altitude it wasn't that hot (in fact my girls got slightly cold at lunch).
Wonderful views of the lake and Rinjani at the top, worth the effort.