Good walking shoes are necessary. While there is a café, if you would like healthier options, pack a lunch. Visitors can take a cruise around Crater Lake but should make reservations online.
July can be very warm and it's also the start of fire season. If you go in November, you may run into snowy conditions and not be able to see the lake. Snow season runs from November to May and roads may be closed at the last minute. It is best to check Crater Lake National Park's Facebook page or website for current weather conditions and road closures.
Any day is a good day, unless seasonal elements close it down.
August is usually the peak of fire season, I suggest planning to go before if you would like to take part in the summer Junior Rangers. Free Ranger-guided snowshoeing runs from Thanksgiving weekend through April, just check road conditions because sometimes it does get cancelled.
If you will be visiting various national parks throughout the year, I highly suggest buying a national parks pass that offers free entry to any parks.