It depends on a lot of variables like time, season, solar conditions, and QTH.
For HF, you can hear a lot of activity on 160m at night. At night and during the day, activity can be heard on 80m and 40m.
20m on the other hand can be worked day and night depending on solar conditions and season.
15 and 12 are usually daylight only bands, but are still a lot of fun to play with.
10m can be open depending on E-skip. 6m is the same, and is a lot of fun to work.
2m isn't a popular phone band for most parts. It's where most people tend to get started though. Your location can also dictate what you hear, especially on VHF or UHF frequencies.
Also, phone and code operations are dictated by band plans and license restrictions. Don't be a lid and operate out of band or in a mode not allowed in a particular frequency range.
And to answer the OP's question; you can buy whatever you want without a license. Just don't expect to use it to transmit. If you do, you will get caught. The penalties can often be extremely expensive.