Most decent media players should be able to play FLAC files. Winamp can (which is what I use for music, the free version works), and if you have the CCCP installed then its FFDShow Audio Decoder should allow FLAC playback in any media player that can use it. Though if you want to play them on a portable MP3 player, chances are it probably won't support FLAC.

To convert FLAC to MP3, there's plenty of simple tools out there to do it. To get really high quality MP3s, though, this is what I do: In Winamp, go to Options > Preferences (or Ctrl+P from the main window), select Output under the Plug-ins section, and select Nullsoft Disk Writer. Hit configure with it highlighted, select Force WAV file under Output File Type, select where you want it to put the files with the options above that, making sure that the Convert to format box is unchecked. (Leaving that unchecked will keep file's sample rate unchanged from the original.) Click OK, close the Preferences window, then open the files you want to convert. With the Disk Writer plug-in selected instead of DirectSound Output, Winamp will spit out uncompressed WAVs instead of playing the files.

Next, to convert the WAVs into MP3s, download the LAME encoder and RazorLame, a user interface for the LAME encoder. (LAME is generally considered the highest quality MP3 encoder available.) Unzip RazorLame into a folder, unzip the LAME files into a folder inside that, then run RazorLame.exe and go to Edit > Options. Click the little folder button in there, find lame.exe in the folder you unzipped it to and click Open, then click OK to save the options.

Now click the LAME button in RazorLame and select the Output Directory on the General tab. Then select the Expert tab, check the Only use custom options box and copy one of the below presets into the text field above it:

--alt-preset standard
--alt-preset extreme
--alt-preset insane

Standard will produce a VBR (variable bitrate) MP3 that averages about 192 kbps. Extreme will also be VBR averaging about 256 kbps. Insane will produce a CBR (constant bitrate) MP3 at 320 kbps, which is the highest quality you can get with MP3. Or, if for some reason you want a lower CBR file, you can clear the preset field, uncheck the box, and play with the other options. When you've got your options selected, click OK. Then click the Add button, open the WAVs that Winamp output (or any other supported audio files you want to encode to MP3), and click the Encode button.

Note that the MP3s that RazorLame outputs won't have any ID3 tags. You'll need to retag the files if you want title/artist info to show up in programs that read from the ID3 tags. Winamp can also do this (View file info or Alt+3 with a file open), or, for large batches, I like to use ID3-TagIT.

tl;dr Find a one-click converter if you don't care about super high quality.

I don't know about all that file conversion stuff, but I'll eat ice cream with you. :3